Boris Johnson is under mounting pressure to recall Parliament following warnings that a no-deal Brexit would cause “incredibly serious” economic harm.

The Prime Minister again asserted that he will deliver on his commitment to leave the European Union on October 31 as his allies sought to play down a confidential Whitehall dossier which detailed the problems posed by a no-deal scenario.

But the leader of the Confederation of British Industry (CBI) said it was impossible to be fully prepared for the disruption that would occur following the UK crashing out of the EU without a deal.

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And shadow chancellor John McDonnell threw his weight behind demands for Mr Johnson to cut short Parliament’s summer recess to face MPs at Westminster.

Mr Johnson used a Twitter post to repeat his message that the UK would “leave the EU on October 31st, invest more money in the NHS and crack down on violent crime”.

He is expected to use his first foreign visits later this week to spell out his commitment to the Halloween deadline in talks with German Chancellor Angela Merkel in Berlin and French President Emmanuel Macron in Paris ahead of the G7 summit in Biarritz at the weekend.

The Prime Minister faced calls to step up efforts to secure a Brexit agreement after the risks posed by a no-deal outcome were outlined in documents on Operation Yellowhammer – the Whitehall codename for no-deal preparations –  obtained by the Sunday Times.

They warned that 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.

CBI director-general Dame Carolyn Fairbairn told BBC Radio 4’s Today programme: “I think we can be prepared, we should be prepared, but I don’t think that takes away from the fact that what Yellowhammer shows, and I think what businesses have been saying for three years, is that the cost will be really significant for our economy and for jobs and that a deal is the number one priority.”

Shadow chancellor Mr McDonnell said Labour wants to see Parliament reconvened “in the next few days”  to drive efforts to avoid a no-deal Brexit.

Countdown to leaving the EU(PA Graphics)

More than 100 MPs have signed a letter demanding Mr Johnson recalls Parliament, and Mr McDonnell said he too backed the move.

“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,” he told Today.

No-deal planning supremo Michael Gove played down Operation Yellowhammer, and Number 10 insiders sought to blame disgruntled ex-ministers for the leak.

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According to the documents, petrol import tariffs would “inadvertently” lead to the closure of two oil refineries, while protests across the UK could “require significant amounts of police resources” in a no-deal scenario.

But a Number 10 source said: “It has been deliberately leaked by a former minister in an attempt to influence discussions with EU leaders.

“Those obstructing preparation are no longer in Government, £2 billion of extra funding (has been) already made available, and Whitehall has been stood up to actually do the work through the daily ministerial meetings.

“The entire posture of Government has changed.”

Michael Gove, the Cabinet minister responsible for no-deal planning, insisted Yellowhammer represented a “worst-case scenario” and said “significant” steps have been taken in the last three weeks to accelerate Brexit planning.

Meanwhile, the Daily Telegraph reported that up to 40 Tory MPs are backing a bid led by former Cabinet ministers Philip Hammond and David Gauke to prevent a no-deal Brexit.

READ MORE: Kirsty Strickland: Recalling Parliament during a national crisis is the only sensible option

Last week the former chancellor and 20 other senior Tories were said to have written to the PM to say his demands to abolish the backstop “set the bar so high that there is no realistic probability of a deal being done”.

Amid speculation that the Government is preparing for an election owing to the Brexit crisis, Jeremy Corbyn will use a speech on Monday to say such a vote would provide a “once-in-a-generation chance” for a change of direction in politics.

On a visit to Northamptonshire, the Labour leader will set out his vision to rebuild Britain and vow to do “everything necessary” to prevent a no-deal exit.