Labour have performed a dramatic shift in policy by committing itself to continued UK membership of the EU single market and customs union during a transition period following the Brexit date of 2019.
Shadow Brexit secretary Sir Keir Starmer says Labour would stick with the "same basic terms" of Britain's current EU membership during the transition, which could potentially last five years.
In an article for The Observer, Sir Keir stated that the party is open to negotiating new single market and customs union term which the UK to agree to on a permanent basis.
Critics of continued customs union membership argue that it would prevent the UK from striking new trade deals with non-EU countries.
Labour promised prior to June's general election 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 not said he would support continued membership beyond the date of Brexit.
Mr Corbyn has come under pressure from certain factions within 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.
According to Sir Keir, the Tory position 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 in The Observer: "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."
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.
Labour MP Chuka Umunna, a leading supporter of the Open Britain campaign against a hard Brexit, said: "This is a most welcome announcement and a significant moment in the Brexit debate so far.
"This will rightly pile the pressure on the Government to put membership of the single market and the customs union at the heart of their negotiating strategy. Anything else will be bad news for our economy, jobs, public services and social justice."
Responding to Sir Keir's announcement, Labour peer and former minister Lord Adonis said on Twitter: "Chances of staying in the EU just rose to nearly 50%. Rejoice, rejoice!"
Liberal Democrat Brexit spokesman Tom Brake said: "This is all spin and no principle.
"Mr Corbyn supported the Conservative Brexit government and is Theresa May's best ally in her attempt to drag Britain out of the world's largest market.
"Judge a party by hard actions, not empty words."