BRITAIN faces “trading off” security at its borders for speedy processing to ease the flow of people and goods into the country post Brexit, a spending watchdog has suggested.

A landmark report has laid bare the scale of the task facing border patrols after leaving the EU - including the potential doubling of passenger verifications and a four-fold increase in customs checks to 255 million.

Holidaymakers are warned to expect “gridlock” and lengthy queues, which may make passengers more vulnerable to attack.

The study, by spending watchdog the National Audit Office, insisted border operations needed to be strengthened to manage “fiscal, security and trade implications” from Britain’s withdrawal from the European Union.

The need to carry out biosecurity and standards checks on livestock, food and other goods could also add to officials' workload.

Ed Davey for the Liberal Democrats said the independent report demonstrated the “stark reality” facing Britain’s border security once the country leaves the EU in March 2019 “What this could mean is gridlock for millions of people returning from their holidays waiting longer for their passports to be checked and a backlog of lorries entering and leaving the UK,” he said.

“This is not what people voted for,” he added.

A Government spokesman said it would “of course” ensure border checks were properly resourced.

While the UK might need to respond with a “system wide transformation at speed and with acceptable levels of risk,” the report pointed to the need for special arrangements regarding the politically sensitive Irish border, noting how the Government wanted to avoid any physical infrastructure, and at crossings like Gibraltar.

The NAO also mentioned “roll-on roll-off” port entries, where increased border controls could limit the flow of goods and people, creating bottlenecks and substantially increased waiting times.

“If these persisted,” the report suggested, “then perishable goods could be damaged and supply chains could be interrupted.”

Referring to balancing short and longer term risk and opportunity, the report stated: “EU exit-related activities will need to be prioritised and some trade-offs made.

“The Government may have to make difficult decisions about possible changes to its risk appetite; for example, by reducing checks on goods if queues build up.”

Labour former minister Chris Bryant, commenting on behalf of the Open Britain campaign group for close ties with the EU, said there was “little chance” HMRC would have the time and resources to expand its customs operation He said: "This points to a nightmare scenario of lorries backed up for miles at Dover, disruption of our food supply, and a massive increase in costs to British businesses which trade abroad.

“That would cost jobs and send our economy hurtling off a cliff.

"And all this to pursue a fantasy of new global trade deals, which in reality cannot replace the trade we do with the European Union and will take years to negotiate."

His view was echoed by Public Accounts Committee chairwoman Meg Hillier, who was sceptical about the Government's ability to respond effectively.

She said: "How the UK manages its borders has been a question of significant concern in the Government's approach to Brexit.

"But this report from the NAO underlines the sheer scale of the task ahead.

"The Government can't even get started on many of these challenges until we have progress on the exit deal.

"Add to that the Home Office's poor track record with projects like e-borders and for all the Government bluster about Brexit it's difficult to see how, practically, it will be able to deliver any of these huge changes in time."

Earlier this week, Philip Rutnam, the chief official at the Home Office, pointed to how the recruitment of 300 extra Border Force staff had begun but admitted that the Army might have to be brought in as an “absolute last resort”.

A Government spokesman said: "We are fully focused on making the UK's exit from the EU, and our new trading relationship with the world, 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.

"We will of course ensure we have the resources we need to continue to run an effective customs, borders and immigration system in the future."