Tony Blair is urging MPs to pass a fresh law to prevent a no-deal Brexit, after a government minister admitted the option will “always” be on the table.

Kwasi Kwarteng’s comments were “a revelation” that made it essential Boris Johnson did not succeed in triggering a pre-Christmas general election, the former prime minister said.

They exposed the real risk of the UK crashing out of the EU at the end of 2020 – the planned end of the transition period – if the Brexit deal is passed in its current form, he argued.

“Kwasi Kwarteng’s words are a revelation that no deal is still a real possibility in the negotiation over Britain’s future relationship with Europe,” Mr Blair warned.

Suspicions that a no deal at the end of 2020 remains the aim of hardline Tory MPs were first prompted when one said it was their reason for backing Mr Johnson’s deal.

Now Mr Kwarteng, the business minister, has put rocket boosters under the fears of “a trapdoor” little more than one year away, by saying: “No deal can’t be taken off the table because it helps with the negotiation.”

He made clear the threat would remain even if the divorce deal passed, telling BBC Radio 4: “It’s always got to be something which may well be a possibility.”

The comments are certain to fuel Labour determination to oppose the trigger motion for the 12 December election Mr Johnson craves, in the vote on Monday.

Mr Blair said: “This statement should transform Labour’s position. To take no deal off the table, it has to be taken off not only in respect of the withdrawal negotiation but also of the subsequent negotiation around the future relationship due to finish by end 2020.”

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And he added: “This is why Labour must not agree to this election ploy until the situation is clarified and dealt with.”

The row came as Sadiq Khan piled pressure on Jeremy Corbyn to be “braver” by telling Leave voters they are “wrong” on Brexit, instead of sitting on the fence.

In an interview with Italian newspaper La Repubblica, the London mayor said “all forms of Brexit are worse than remaining in the EU”.

“I’d like the Labour Party to be braver and provide leadership on this issue,” Mr Khan said, ahead of vying for a second term at City Hall in May.

“Sometimes, saying to people who may want to leave the EU, ‘Listen, I respect you, but let me explain to you I think you’re wrong and why I think we should campaign to remain in the EU’.”

Mr Khan added: “That’s the conversation we should be having rather than trying to be all things to all people.”

The new divorce deal retains the clause from its rejected predecessor allowing an extension to the transition period until December 2022.

However, the risk of a no-deal Brexit will only be delayed, not removed, because of Mr Johnson’s insistence that it must end at the close of 2020.