In a major policy shift, Labour's Brexit spokesman Keir Starmer wrote in the Observer that continuity immediately after Brexit would "avoid a cliff edge for our economy".
"Labour would seek a transitional deal that maintains the same basic terms that we currently enjoy with the EU," Starmer wrote.
"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 said, meaning unimpeded immigration from the EU could continue.
But he hinted that Labour could aim to retain some form of membership of the customs union even once the transition period has elapsed.
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"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," he said.
Starmer today dismissed "fanciful and unachievable" proposals set out by Brexit minister David Davis.
He also warned about the government's "ideological obsession" with taking Britain out of all of the EU's structures when the country leaves the bloc in March 2019.
The Labour party are in a powerful position because Prime Minister Theresa May's Conservatives lost their majority in elections in June and have to govern with the support of Northern Ireland's Democratic Unionist Party.