Chancellor Philip Hammond has signalled that he is ready to vote with Labour to block a no-deal Brexit under Boris Johnson.

In a dramatic warning to the Tory leadership front-runner, Hammond declared that a “disruptive” exit from the EU would mean a £90bn “hit” to the British economy and vowed to do everything he could to stop it.

Asked directly by the shadow chancellor if he would join the Opposition in fighting a no-deal exit in parliament, Hammond gave his strongest hint yet that he was ready to do so.

“It would be wrong for a British government to seek to pursue no deal as a policy and I believe that it will be for the House of Commons – of which I will continue proudly to be a member – to ensure that that doesn’t happen,” he said.

In what may be his last ever Commons question time as a Treasury minister, Hammond rammed home his point that both Johnson and Hunt’s plans relied on his plans to borrow an extra £26bn – money they wouldn’t have if the UK quit the EU without an agreement.

“I have no doubt whatsoever that in a no-deal exit we will need all of that money and more to respond to the immediate impacts of the disruption of a no-deal exit, and that will mean there is no money available for longer-term either tax cuts or spending increases.

“But let me go further, the government’s analysis suggests that in a disruptive no-deal exit there will be a hit to the exchequer of about £90bn. That will also have to be factored in to future spending and tax decisions.”

The chancellor added that “leaving with a no-deal exit will be bad for the UK, bad for the British economy, bad for the British people”.

The government has a current working majority of just four and that could go down to three if it loses the Brecon by-election next month.

Shadow Chancellor John McDonnell suggested that Hammond would be one of a string of Tory MPs who would join a new cross-party effort to block no-deal would take place in October, just days before the UK’s extended EU membership expires on Halloween.

“You saw today a vehemence in Philip Hammond that you don’t often see. He’s an influential voice in the Tory party and he has a number of people who will line up with him,” he told HuffPost UK.

“He was ferocious in his attack on no-deal, emphasised the £90bn cost to the economy and was making it clear that he would do everything he could to oppose it.

“Certainly if he is on the backbenches he will be one of the most influential Tory backbenchers to align themselves with our own position of opposing no-deal.”

Although Johnson has said he may ignore MPs, several Commons grandees are working on a plan to stage an emergency debate to block a no-deal, as well as using other novel parliamentary conventions.

“When we [MPs] get back in September, October, largely October, there’s a short window of opportunity. I think there’s an issue about the moral authority of parliament here,” McDonnell said.

“If there’s vote after vote after vote indicating that parliament does not support a particular proposal it’s very difficult to see a prime minister being able to exercise authority.”