British foreign secretary and leading Brexiteer Boris Johnson leaves the polling station after casting his local government vote at Charing Cross Library, London yesterday. Photo: Stefan Rousseau/PA Wire

Britain will be unable to leave the European customs union before 2023 ministers have been told, meaning the delay could be exploited by Remainers to thwart Brexit.

In a briefing to the British Cabinet's Brexit sub-committee earlier this week, senior civil servants said highly complex new technology which will be needed to operate Britain's borders after Brexit may not be ready for another five years.

UK Prime Minister Theresa May has asked officials to carry out more work on the two options being considered by the Government to replace the customs union: a customs partnership and a so-called "maximum facilitation" plan.

The disclosure was made despite claims yesterday by David Davis, the Brexit minister, who told MPs he was "100 per cent" sure Britain will have left the customs union by the end of the transition period on December 31, 2020.

The expected delays mean that whatever option the Cabinet decides on, the country will have to stay in a form of customs union for four years after leaving the bloc in March 2019.

Brexiteers have expressed serious concerns that Remainers in Parliament will use the delay to try to keep Britain in the customs union and possibly even the single market.

One senior Brexiteer said: "There are genuine concerns this delay will lead to the UK staying in the customs union permanently.

"Regardless of that, if we are still in the customs union by the next general election in 2022 it will cause a catastrophe at the polls because we will not have delivered Brexit and voters will not have seen any benefits of leaving the EU."

The government is yet to agree a position on the customs relationship with the EU after a meeting of the Brexit sub-committee broke up without a decision on Wednesday.

Mrs May's preferred option of a customs partnership with the EU was rejected by a majority of the 11-strong committee, and the prime minister was warned yesterday that there must be no attempt by her to revive the policy.

Brexiteers fear she will present a "customs partnership 2.0" plan to the committee later this month in the hope of winning over the Remainers Gavin Williamson, the defence secretary, and Sajid Javid, the home secretary, both of whom sided with Leavers during the heated discussion.

The alternative being discussed by ministers is a "maximum facilitation" plan, or "max fac" for short, which will harness new technology and trusted trader schemes to avoid a hard customs border in Ireland.

The EU has rejected both ideas out of hand, giving succour to Remainers who argue that staying in the customs union is the only viable solution if Britain is to solve the Irish Border problem.

Mrs May has been told by Julian Smith, the chief whip, that the government is likely to lose a crucial vote in parliament in the coming weeks calling for ministers to make a customs union with the EU one of their Brexit negotiating objectives.

Tory rebels led by Dominic Grieve, the former attorney general, have tabled an amendment to the Customs Bill designed to keep the UK in a customs union.

One source said: "The whips have informed the Prime Minister that the Remainers have the numbers to beat the Government on customs votes in Parliament."

It is now possible Mrs May will try to delay the vote to give her more time to win over the mutineers in her party. Mrs May has repeatedly said Britain will leave the customs union and not be a member of a customs union with the EU after Brexit.

Her struggle to find an alternative became more complex after Olly Robbins, the UK's top Brexit civil servant, delivered a "frank" assessment of the options on Wednesday.

One Whitehall source said: "The estimate is it would take five years to get the technology up and running. Olly Robbins said it could be done by 2022 at a pinch, but most people think even five years is an optimistic estimate.

"The Remainers will try to use it to keep us in the customs union for good, and they will then argue that we might as well be in the single market as well."

Brexit-supporting cabinet ministers, including foreign secretary Boris Johnson and environment minister Michael Gove, believe that they "killed off" Mrs May's favoured solution of a customs partnership during Wednesday's meeting after it was opposed by a 6-5 majority.

Government sources have suggested that "elements" of the customs partnership idea, which involves collecting tariffs on behalf of the EU before companies claim a rebate, may remain in a reworked model which is expected to be presented to the same committee next week.

Meanwhile, yesterday's local elections were considered a test of the public mood.

Voting was taking place to fill more than 4,000 seats on 150 local councils in towns and cities across England, including all of London's 32 boroughs. Scotland, Wales and Northern Ireland were not holding elections.

The Conservatives were braced for losses amid anger over unsteady Brexit negotiations, an explosive immigration scandal and years of public spending cuts that have seen local officials close libraries and slash services.

Irish Independent