Labour is committing itself to continued UK membership of the EU single market and customs union during a transition period following the official Brexit date of March 2019.
In a dramatic policy shift, the party's shadow Brexit secretary Sir Keir Starmer has announced that a Labour government would abide by "the same basic terms" of Britain's current EU membership during the transition, which some observers expect to last as long as four or five years.
And in an article for The Observer, he made clear that the party is open to the possibility of negotiating new single market and customs union terms which the UK could sign up to on a permanent basis.
At June's general election, Labour promised to seek to "retain the benefits" of the single market and customs union as part of a "jobs-first" Brexit, but leader Jeremy Corbyn has so far stopped short of committing to continued membership beyond the date of Brexit.
He is coming under pressure from some parts of the party to adopt a more pro-EU stance, with a new group backed by former shadow health secretary Heidi Alexander and Wirral South MP Alison McGovern calling for a policy of "unequivocal" support for membership of the single market, customs union and European Economic Area.
Sir Keir told The Observer that the Tory position, set out by Chancellor Philip Hammond and International Trade Secretary Liam Fox, of taking the UK outside the single market and customs union for the transition period would be "unnecessary and a highly risky path to take".
The shadow Brexit secretary wrote: "Labour would seek a transitional deal that maintains the same basic terms that we currently enjoy with the EU. That means we would seek to remain in a customs union with the EU and within the single market during this period. It means we would abide by the common rules of both."
And he added: "We will always put jobs and the economy first. That means remaining in a form of customs union with the EU is a possible end destination for Labour, but that must be subject to negotiations.
"It also means that Labour is flexible as to whether the benefits of the single market are best retained by negotiating a new single market relationship or by working up from a bespoke trade deal."
Mr Corbyn's office confirmed that the proposals set out by the shadow Brexit secretary had been agreed with the party leader and had the status of official policy.
Sir Keir said that under Labour, the transition period before the final shift to a new UK-EU relationship would be "as short as possible, but as long as is necessary" and would be time-limited in order to prevent it becoming "a kind of never-ending purgatory".
A final deal would have to involve "more effective management of migration" while retaining the benefits of the customs union and single market as part of a "strong and lasting new relationship", he said.
Critics of continued customs union membership argue that it would prevent the UK from striking new trade deals with non-EU countries.