Would-be PM Andy Burnham was left humiliated on BBC Newsnight after failing to answer a really simple question – sparking a monumental backlash online. The Greater Manchester Mayor has confirmed that he will try to depose Sir Keir Starmer and join the race to become prime minister if he wins the crunch Makerfield by-election on June 18.

The 56-year-old has faced repeated questions about what he would do if he made it to No 10, with his team confirming he would stick to Rachel Reeves’s tough fiscal rules to safeguard the UK economy. But when asked by BBC Newsnight presenter Victoria Derbyshire what the Chancellor’s fiscal rules were, Mr Burnham point-blank refused to answer. “I’m not going to go through a discussion like an exam on the fiscal rules,” he snapped back.

“Why not?” Derbyshire demanded to know.

“I know what the fiscal rules are,” he replied, before Derbyshire asked: “Remind us what they are.”

Mr Burnham dodged the question, saying: “I’ve been very clear about it. I’ve set out a plan that is within those rules so that we keep the discipline.”

Referencing his previous controversial comments about not wanting Labour to be “in hock to the bond markets”, Mr Burnham said: “Nothing I’ve said at any point in this campaign, or before this campaign, has been about ignoring the bond markets or saying that they don’t matter.”

His response has been widely panned online, including by his biggest political opponents.

Tory Shadow Chancellor Sir Mel Stride said: “Andy Burnham clearly doesn’t know what the fiscal rules are.

“He made a big statement that he would stick to them but doesn’t seem to have acquainted himself with what he was committing to.

“And he wants to be prime minister.”

And Robert Jenrick characterised Mr Burnham’s position as “I’m sticking with Reeves’s fiscal rules, but I don’t know what they are”.

The Reform UK Treasury spokesman added: “It’s an insult to the British people that this is the best Labour can offer.”

A poll this week gave Mr Burnham a 10-point lead in the by-election over Reform UK candidate Robert Kenyon, with Rupert Lowe’s Restore party appearing to split the vote on the Right.

The Greater Manchester Mayor admitted on this week’s BBC Question Time that he would make a bid for the Labour leadership if he wins the by-election.

Asked about his intentions if he wins the seat, he said: “I’m not somebody who gets ahead of myself. I can’t do anything unless I’m lucky enough to get the support of people here.

“But if I get your support, I would seek to represent you at the highest possible level and give this constituency maximum power and influence.

“I think Wes Streeting seems to have launched a leadership contest, so if that is running, I would seek to join it. But I’d have to persuade members of the Parliamentary Labour Party to do the same.”