Operation Postmaster was, according to leading military historian Damien Lewis, perhaps the biggest gamble of Winston Churchill’s entire political career. Utterly deniable in the event of failure, it involved a small team of raiders navigating a Brixham Trawler 3,000 miles across the Atlantic Ocean to a tiny Spanish island off the West coast of Africa.

There they engaged in what amounted to blatant piracy, boarding and stealing three vessels, an 8,000-ton Italian liner, a German tug and a pleasure yacht, in flagrant breach of Spain’s Second World War neutrality, before rendezvousing with the Royal Navy. The mission involved the blackmail of the island’s Spanish governor – photographed naked, showering his local mistress with a watering can – an adapted trawler armed to the teeth with hidden guns, and a motley all-volunteer crew posing as Swedish tourists.

And the “cutting out” operation in January 1942 was conceived, in part, by James Bond creator Ian Fleming. It would be death or glory. No wonder the staid British military leadership and political hierarchy wanted nothing to do with it. If this all sounds more like a Guy Ritchie film than a serious undercover mission, it’s both. Lewis’ stirring 2014 account was adapted earlier this year into a riotously entertaining and deliciously-titled new film, The Ministry of Ungentlemanly Warfare. And it’s still delighting Amazon Prime viewers.

Featuring Henry Cavill as British special forces pioneer Gus March-Phillipps, and Jack Reacher star Alan Ritchson as his Danish colleague Anders Lassen, later the only member of the British SAS to win the Victoria Cross, it’s a classic Ritchie production, combining the kind of pitch-black humour, brilliantly over-the-top action sequences and razor-sharp editing that have come to define the Lock, Stock and Two Smoking Barrels director’s best work. The film also includes memorable performances by Alex Rider star Alex Pettyfer, playing real-life Commando Geoffrey Appleyard, and Mexican actress Eiza Gonzalez as Special Operations Executive agent Marjorie Stewart, who went on to marry March-Phillipps.

“Seeing them at work, I realised somehow the cast had recreated that maverick, do-the-unthinkable, think-the-unthinkable, let’s-all-have-fun-while-we’re-about-to-diestyle atmosphere on the set,” Lewis says.

“There was real chemistry. It was like a real special forces unit, a ‘Chinese parliament’ as the SAS calls it, where everyone throws ideas in. It doesn’t matter how unthinkable, because they have to do the unthinkable.”

Operation Postmaster itself, lightly fictionalised on screen, is a case in point.

As Lewis marvels: “To this day, I cannot understand how Churchill got this plan through, it just beggars belief. I mean, on the one hand, I take my hat off to his sheer balls.

“But the downside was incalculable, that’s why there was so much opposition.That’s why so many times throughout Operation Postmaster, the military leadership tried to torpedo it.The raiders would be violating that neutrality of Spain by sinking or stealing German or Italian vessels – Spain could have closed the Mediterranean to the British.

“But Churchill backed it to the hilt, which was, in my view, the biggest gamble of his political career. If Postmaster had gone wrong, Spain would be forced to join the war.”

Having formed the Special Operations Executive in 1940 to “set Europe ablaze”, little excited the PM more than the idea of dashing commando-style raids, later described by Churchill in his wartime memoirs as “butcher and bolt” operations.

“Churchill realised the war was going to be fought as no conflict had been fought before – no holds barred, everything on the table,” says Lewis. “The rules of war had been torn up by Hitler. In fact, you had to out-Hitler Hitler, and the SOE was set up to do all the things you weren’t allowed to do: bribery, corruption, money laundering and smuggling. I mean, you name it, they did it.

“Churchill said, ‘Leave a trail of German corpses in your wake and ensure no enemy can sleep soundly at night’. He knew if you could strike behind their lines killing anybody, no matter what their rank or, even better, capturing and spiriting them away, it would strike terror into their hearts. Morale would collapse.”

He also understood that even “pinprick, mosquito raids”, as Churchill called them, would be worth their weight in propaganda. “It showed the ‘British Bulldog’ still had bark and the ability to bite back, even in our darkest hour.”

By contrast, the powers-that-be believed war should be “played by the rules”. “Most of the senior military commanders and political leaders had been schooled in the First World War and they still viewed things as static trench warfare,” Lewis, 58, a father of four who lives in the West Country, continues.

“But more importantly, they believed you could wage war in a gentlemanly, chivalric way, which you couldn’t. And the more successful the SOE and, later, the SAS were, the more they were resented because they were proving the naysayers wrong.” The SOE became known by its detractors as all sorts of things: “The firm”, “The Racket”, “The Baker Street Irregulars”, and even “The Ministry of Ungentlemanly Warfare”, “because that’s what it was and that’s what British officers shouldn’t have been doing,” Lewis explains.

“And in typical fashion, as maverick, freewheeling, freethinking individuals, they embraced their nicknames. I think that’s rather good.”

Having been approached by an SAS contact with a story that needed to be told “for posterity”, Lewis revealed the incredible tale in his bestselling 2014 account, Churchill’s SecretWarriors, now retitled to match the film, which introduced the exploits of March-Phillipps and his team. Back then, there were still half a dozen veterans alive.Today just one remains, Jack Mann, 98, although he did not take part in Postmaster.

As founder of No 62 Commando, also known as the Small Scale Raiding Force – precursor to today’s SBS – March-Phillipps created the template for modern Special “They were the original deniable operatives. It was the kind of stuff we now almost accept as a given, but back then this was an unthinkable way to behave,” says Lewis.

“They were posing as Swedish tourists on a pleasure cruise knowing that, if captured, the government would deny any knowledge of them and they would be treated as spies. That whole package was just unheard of. No one had done it and it was deeply, deeply reprehensible to an awful lot of people on high. Now it’s a given these things take place.

“We have bespoke units in the shadows that do these things.”

March-Phillipps’ boss at the Admiralty was Fleming, who went on to create 007, infusing his fictional super-spy with much of his former comrade in arms’ character.

“Fleming came up with various brilliant schemes and they were all blocked by their political and military masters, because no one liked these people, no one liked what they were doing, no one liked their different way of thinking,” says Lewis. “It was only March-Phillips and Fleming’s perseverance that got Postmaster off the ground.”

Perhaps the greatest character in the saga is Anders Lassen, the scion of a wealthy Danish family who had made their money in tobacco. He was overseas working as a merchant seaman when the Germans invaded his country.

“He volunteered for the British army but they basically decided he was just too wild and unruly,” chuckled Lewis. “So they passed him to the SOE. One of the first things Lassen does is petition the War Office for the use of the bow and arrow as the ideal weapon: a silent killer that never runs out of bullets – you could make arrows from anything. And the War Office did a study, it’s all in the archives, incredibly, and they came back saying, ‘Yes, you’re right. However, it’s too inhumane a weapon for use’.

“So on paper they didn’t let Lassen use it. But of course, that didn’t stop him at all. With his blonde hair and foreign accent, Lassen kept getting arrested by the Home Guard during training in Dorset as a suspected German spy. Eventually he was nicknamed the ‘Danish Robin Hood’.”

The irrepressible Dane would become March-Phillipps’ trusted second in command. Having met US actor Ritchson, 41, while they were filming aboard the Imperial War Museum’s floating museum, former Royal Navy cruiser HMS Belfast, Lewis reveals: “He’s something like 6ft 4ins and equally wide. I asked what it was like playing Lassen who was slim, less than 6ft tall and had a strong Danish accent.

“He told me, ‘I might be twice his size, but I can tell you one thing, I’m bloody good with the bow and arrow’. And he was.”

Lewis believes Lassen was probably responsible for more enemy fatalities than any other similar operative in the entire war. “In my view, you can have good and bad psychopaths, and Lassen was a good one.”

Later, the Small Scale Raiding Force was taken to North Africa, trained as SAS, and became the SBS under General Dwight Eisenhower’s command to take part in Operation Husky, the Allied invasion of Sicily in July 1943. By then, March-Phillipps would be dead. Having received the Distinguished Service Order for his part in Operation Postmaster, and married, he was killed aged 34 in September 1942 during a failed raid on the coast of occupied France on part of what later became Omaha Beach in the D-Day landings. Lassen died aged 24 in April 1945 in Italy.

Posthumously awarded the Victoria Cross, he was the only non-Commonwealth recipient of the honour – and, incredibly, remains the only member of the British SAS to this day to receive a VC. He is one of only three statues at the SAS regimental headquarters in Hereford today, the others being SAS founder Sir David Stirling, and his successor, Paddy Mayne.

Despite their deaths, a potential sequel might massage the historical record to allow the actors to return, featuring later SOE ops.

Lewis smiles: “Henry Cavill held his hand out to me and said, ‘Without you, we wouldn’t have a film and I’m so honoured to be playing this man’. When I told him his character dies, he said he wouldn’t allow it!”

Otherwise, however, the self-described “popular historian” is happy with Ritchie’s adaptation of his book, even the trademark humour. “If this film gets seen by millions of young people – which it already has – and just a fraction go and buy the book or research the subject, that’s music to my ears, because it means they’re finding out about Second World War history,” he adds.

“We have a war going on in Europe. We’ve got Putin trying to mess with democracy – the freedoms for which these people sacrificed so much. So if we can keep getting that message out, I’m all for popularising history as much as we can.”

The Ministry of UngentlemanlyWarfare is available now on Prime Video. Damien Lewis’s book of the same name is published in paperback by Quercus, priced £10.99. Visit expressbookshop.com or call 020 3176 3832