Boris Johnson has rejected demands for parliament to be recalled "within days" amid a row over a leaked Whitehall dossier warning that a no-deal Brexit would trigger months of chaos.

Both Jeremy Corbyn and John McDonnell threw their weight behind a plea from more than 100 parliamentarians for MPs to cut short their summer break to tackle the looming threat of a disorderly exit from the EU in the autumn.

It comes after Downing Street pushed back against a secret government document, which said Britain will be hit with a three-month "meltdown" at its ports, a hard Irish border and shortages of food and medicine if the UK leaves without an agreement.

Amid a deepening row, more than 100 MPs wrote to the prime minister at the weekend to demand parliament returns early in order to tackle the "national emergency" posed by a no-deal departure from the EU.

Mr McDonnell, the shadow chancellor, told BBC Radio 4's Today programme: "I agree - I think it is a good initiative by this group of MPs to say that we need to get back into parliament.

"We are facing a critical issue here and should be debating it in parliament."

Mr McDonnell, an ally of the Labour leader, said he agreed that parliament needs to be reconvened "in the next few days".

He added: "There is a need now to bring MPs back together again because we need time now to really have a proper debate and discussion about this matter."

But Downing Street rejected calls for parliament to be recalled to discuss the possibility of a no-deal Brexit.

"The House of Commons agreed the date it would rise for summer recess, as well as its return on September 3 and this was passed by a majority of close to 200 MPs," the No 10 spokeswoman said.

It comes after cross-party MPs - including several Conservatives - wrote to Mr Johnson to warn that the country was "on the brink of an economic crisis" and to urge the recall of the Commons.

The group said: “We face a national emergency, and parliament must be recalled now in August and sit permanently until 31 October, so that the voices of the people can be heard, and that there can be proper scrutiny of your government.”

Labour has reached out to other opposition parties but it would refuse to support a caretaker government led by anyone other than Mr Corbyn, Mr McDonnell said, following suggestions of other candidates such as Tory grandee Ken Clarke.

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Westminster is braced for Mr Corbyn to table a no-confidence vote in the prime minister when parliament returns in September.

However Mr McDonnell said Labour was also locked in talks with opposition parties about other mechanisms to thwart a no-deal scenario - and suggested there was a "gaining majority" in parliament for another referendum.

Mr McDonnell said: "The reason it has dragged on in the negotiations in the past to find out what is credible and what is not, and the debates in parliament, is because the options that have been put forward have not been credible by Theresa May, and they have not been sound, even credible within the European debate that's taking place.

"So I think there is a real route through now. The most important thing now is to block no-deal.

"I think the opposition parties and reaching across to Conservative MPs as well, I think there is still a majority in parliament to say to Boris Johnson that we will not accept a no-deal.

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"I think there is a gaining majority now to say we have to go back to the people in some form of public vote and that is - in my view - a referendum, that's the Labour party policy now, and therefore we are narrowing down the discussion into what then goes on the ballot paper."

Meanwhile, Mr Corbyn used a speech on Monday to say a snap general election would provide a "once-in-a-generation chance" for a change of direction in politics.

On a visit to Northamptonshire, the Labour leader said: "As we look towards the return of parliament in September this country is heading into a political and constitutional storm.

"It is the Conservative [arty's failure on Brexit and its lurch to the hard right that has provoked the crisis our country faces this autumn.