Britain’s most senior military officer has promised the army will double its lethality in three years, but backed away from the idea of endorsing conscription in his first speech following the general election.

Adm Sir Tony Radakin, the chief of defence staff, said that the British army could justify extra investment if its effectiveness improved, though he stopped short of asking for more money while Labour ministers reviewed the public finances.

The head of the armed forces said the army was “focused on doubling the fighting power of land forces by 2027 and to triple it by the end of the decade” and argued that politicians would respond if land forces could do more for less.

Labour has begun a strategic defence review after the general election, at a time when the size of the British army is at its smallest for 300 years and when there are calls to boost defence spending in response to Russia’s invasion of Ukraine.

But while the government has agreed to lift defence spending from the current level of 2.32% of GDP – £64.6bn – to 2.5%, it has not yet committed to a firm timetable. Ministers want to look at the nation’s books before deciding what is possible.

“Politics responds to positive reasons to invest,” Radakin told the Land Warfare conference, and argued that “the ambition grows” the more the military is able to “make the case for a stronger and more capable army”.

However, Radakin did not call for a return to national service, a surprise election promise made by the defeated Conservatives, and said the UK’s national security did not depend on an army enlarged by young conscripts.

“Poland is doubling the size of its army over the next decade. The Baltic and Nordic states are talking about mass resilience and conscription. That is understandable. They border Russia. The threat is close. Our geography is different,” Radakin said.

Several months before the election, in January, the former army chief Sir Patrick Sanders suggested that a form of conscription to create a citizens’ army on the Nordic model may be necessary, arguing that the UK was in a “pre-war” moment.

Sanders has since retired from the military and Radakin and his successor, Sir Roly Walker, are carefully aligning the army’s ambitions with Labour. The party dismissed Sunak’s national service plan as a gimmick during the election campaign.

However, the British army’s full-time strength has fallen below the previous government’s target of 73,000, while considerable supplies of munitions and equipment have been donated to Ukraine.

Radakin said the army had “near-term financial challenges to work through”. There had been “historic under-investment” and there were “deficiencies in people, equipment, stockpiles, training and technology”.

Military chiefs believe that the combined threat from Russia, even though it is embroiled in a costly war in Ukraine, China and possibly Iran and North Korea is increasing and will become sharper by 2027 or 2028.

It is not clear how increasing lethality will be measured, but the army believes it can make better use of armed drones – a mainstay of the battlefields in the east of Ukraine – and deploy advanced technologies, notably artificial intelligence.

At the same time, Ukraine’s military has demonstrated it has been able to make effective use of weapons the UK and other Nato countries were retiring, leaving the army to consider whether older munitions are being rendered obsolete too quickly.

Radakin said it was important not to over-exaggerate any threat posed by Russia, and repeated a Ukrainian figure that the invaders had lost 550,000 killed or wounded since the full-scale invasion began in February 2022.

It would take a while for Moscow to rebuild, he argued. “It would take Putin five years to reconstitute the Russian army to where it was in February 2022, and another five years beyond that to rectify the weaknesses that the war has revealed.”