The UK border could be left vulnerable from the first day of Brexit due to the Government’s “reckless” approach to planning the withdrawal from the EU, an influential Commons committee has said.

Cross-party MPs on the Public Accounts Committee (PAC) warned the current approach to preparing for Brexit’s immediate impact was “not fit for purpose” as officials were too reliant on the possibility of a transition period to iron out problems with customs and immigration.

In a critical new report, the committee found vital IT systems might not be ready in time for March 2019 – the official exit day – and there was no planning for major new physical infrastructure such a lorry parks and customs posts to cope with the coming changes.

The Treasury was also told to speed up payments for Whitehall departments preparing for Brexit, as officials were still negotiating for vital cash on a case-by-case basis, the report said.

Labour chairwoman Meg Hillier accused Government departments of an “over-reliance on wishful thinking”, which could leave Britain open to border chaos with enormous queues at Dover and other checkpoints, huge disruption to trade and shortages of customs staff.

It comes after the spending watchdog recently published a critical report warning border officials could struggle to cope after Brexit as customs declarations are set to soar by 360% and immigration checks will rise by 230%.

One of the committee’s main concerns is that officials are assuming that managing the border will not change on exit day – and they are putting their hopes on a transition period to develop new systems and infrastructure.

It also found the new Border Planning Group had been stymied in preparing for challenges around the 300 crossing points at the Irish border as it needed political progress on the issue to advance.

Ms Hillier said: “Government preparations for Brexit assume that leaving the EU will present no additional border risks from freight or passengers. It has acted – or rather, not acted – on this basis.

“This approach, in the context of what continues to be huge uncertainty about the UK’s future relationship with the EU, might generously be described as cautious.

“But against the hard deadline of Brexit it is borderline reckless – an over-reliance on wishful thinking that risks immediately exposing the UK to an array of damaging scenarios.”

The committee remains deeply concerned over the “threat of chaos” if HMRC’s new customs system is not ready in time for Brexit, as customs declarations are set to rise fivefold to 255m a year.

Ms Hillier said: “We were deeply concerned by the lack of progress on this back-up plan. It is now alarming to note such weak contingency planning extends across Government departments.

“The volume of traffic at the border under current arrangements is substantial - in 2016, around 300 million people and 500 million tonnes of freight crossed it.

“After Brexit, the number of decisions required about people or goods crossing could more than treble and more than quadruple respectively.

“These figures should concern all in Government and in our view its current approach is not fit for purpose.”

Her concerns over potential border chaos were echoed by committee member Layla Moran, who warned that “mismanagement and blunder” were risking Britain’s future.

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The Liberal Democrat MP said: “Massive new pressures to the UK border are being ignored and the whole issue is seemingly being kicked into the long grass.

“It is appalling that major issues that we know will arise from Brexit are going ignored. The people must be given a chance to escape this mess and exit from Brexit.”

The new customs system should be on track by January 2019, which will be able to cope with the vast increase in demand, according to HMRC.

A Government spokesperson said: “We are fully focused on making the UK’s exit from the EU a success.

"We have set out proposals for an ambitious future trade and customs relationship with the EU and we will be setting out proposals for the future immigration system in due course.