AS many as 5,000 extra customs staff might be needed to cope with the demands of Brexit, Britain’s tax authority has warned, as it said introducing a single operation for carrying out border and tax checks could cost the taxpayer up to £800 million.

HM Revenue and Customs told MPs it was looking into creating a Singapore-style system that could make trade simpler for businesses by allowing them to get their tax and border assessments done in one place.

Jon Thompson, the body’s Chief Executive, said he was investigating the "business case" and had hired the team which delivered the Singapore project but stressed the Treasury would have to stump up between £500m and £800m to get it off the ground.

Speaking to the Commons Treasury Committee, he explained how it was "conceptually possible" for the Government to bring together 26 different organisations into a "single window," where businesses could do their border checks in one go.

"We need to be transparent with you; that is a mega project,” Mr Thompson declared. “You need to be thinking about that as a project that costs somewhere between £500 to £800 million.

"It would take five to seven years to implement. We have been asked to look at whether there is a business case for that because there would be a noticeable change to GDP.

"It would make it much smoother to import and export if you only had to go to one place instead of multiple different Government departments."

The committee hearing came after the Government revealed its "future partnership paper" last month, which contained two potential options for long-term custom arrangements.

One was a more ambitious option, described as "innovative and unprecedented", which would see Britain "mirroring" Brussels tariffs for goods that would eventually enter the EU to avoid the need for a UK-EU customs border.

The other was a "streamlined" approach, involving the UK negotiating agreements with the EU to reduce trade barriers and harness technology to avoid long queues at ports.

HMRC told MPs that, using a "crude estimate," it could require an additional 3,000 to 5,000 people by the March 30 2019 to meet the increased custom demands posed by Brexit.

Jim Harra, HMRC's Director General of Customer Strategy, said the organisation would have to deal with an additional 130,000 new companies after Brexit that import and export within the EU but do not currently come into contact with British customs.

Meanwhile, Lord McConnell, the former First Minister, ahead of a speech at Glasgow University on Thursday night, urged the Governments in London and Edinburgh to “get on with the job” and take the changes from Brexit more seriously.

"Brexit will be the biggest constitutional change in the UK since devolution. The UK and Scottish Governments have to take the scale of that change more seriously.

"They need to stop positioning themselves for a future[Scottish independence] referendum and, instead, set out their positions on the devolution of powers repatriated from Brussels and the way in which they will work together in certain key areas in the future.”

The Labour peer told the PoliticsHome website: "More speeches and stand-offs will not resolve these issues, negotiations and attention to detail will. I urge both Governments to get on with the job and make it happen."

In other developments:

*Pete Wishart, the SNP’s Shadow Commons Leader, during Business Questions tore into the Tory Government, accusing it of behaving little better than a "dysfunctional tinpot dictatorship" over Brexit legislation after an "absolute stinker" of a week for parliamentary democracy;

*Rex Tillerson, the US Secretary of State, in London for talks with Boris Johnson, the Foreign Secretary, insisted America would be a "steadfast ally" as the British people faced the “unique challenges” of Brexit and

*John Bercow, the Commons Speaker, made clear he would allow votes on difficult Brexit issues for the Government, insisting "nothing is too toxic" as he acknowledged MPs were concerned about the issues of citizens' rights, economic arrangements and the use of so-called Henry VIII powers to allow ministers to amend laws without full scrutiny.