Prime Minister Theresa May revealed plans today for Britain to temporarily align with EU customs rules after Brexit as a fall-back option to resolve the Irish border problem, while also in a compromise with eurosceptic ministers setting a limit to 2021.
The so-called backstop proposal was published after a last-minute wrangle over the wording with Brexit Secretary David Davis, who was reported to consider resigning unless it included a time limit.
Brussels has proposed that Northern Ireland stay aligned with the EU until another way is found to avoid customs checks with the Republic of Ireland after Brexit.
London has rejected this plan as unacceptable, offering instead a "temporary customs arrangement" for the whole of Britain that would "maintain the status quo for traders in respect of customs processes".
May has been clear that she does not want this option, hoping instead to resolve the border issue with a wider trade deal between the EU and Britain.
But she agreed in December to the need for a plan B if this is delayed or does not happen.
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"The UK is clear that the temporary customs arrangement, should it be needed, should be time limited," said the document, which has been sent to Brussels.
It adds: "The UK expects the future arrangement to be in place by the end of December 2021 at the latest." Britain is leaving the European Union in March 2019 but has agreed a transition period where its position remains largely the same until December 2020.
The EU's chief negotiator, Michel Barnier, welcomed the publication of the plan, saying on Twitter that it would be examined with three questions in mind. "Is it a workable solution to avoid a hard border? Does it respect the integrity of the SM/CU (single market / customs union)? Is it an all-weather backstop?" he said.
The publication came after two days of swirling rumours that Davis was planning to resign over the document.
His chief of staff, Stewart Jackson, tweeted that there had been a "helpful dialogue" and the paper now included more detail on "the time limited nature of our proposal".
One pro-European MP in May's Conservative Party said: "This is a clear compromise to keep David from resigning, but it does undermine Theresa hugely."
Yesterday reporters asked Davis if he would quit if the backstop did not have his explicit approval, replying: "That's a question I think for the prime minister to be honest."