The shift in Labour policy is seen as a nudge towards a Soft Brexit position.
The majority of Labour MPs responded positively to Mr Corbyn’s speech, although Frank Field warned that reneging on the spirit of the referendum vote might come back to bite the party.
Mark Littlewood, the director general of the Institute of Economic Affairs, said: “Membership of the customs union is a choice the Government has to make and it is a very straightforward one.
“Leaving the customs union will allow the UK to pursue its own trade deals; staying in a customs union means there will be no scope for UK trade deals. The former is a far more attractive option.
“Remaining in some form of customs union will make it almost impossible for the UK to make the most of the opportunities Brexit has to offer.
“It would prevent the UK from reducing barriers to trade with the rest of the world, whether by signing new deals or simply lowering these barriers unilaterally.
“There is no meaningful difference between ‘a’ customs union and ‘the’ Customs Union. Both would require the UK to impose the EU’s common external tariffs (and other barriers) on imports from the rest of the world, and thus prevent the UK from taking back control of trade policy.
“What’s more, a customs union is not the only solution to the challenges that lie ahead.
“The UK could keep any additional frictions with the EU to a minimum with a new and comprehensive free trade agreement, including alternative customs arrangements.
“While remaining in some form of customs union might have some benefits such as preventing disruptive border checks including in Ireland, and allowing tariff-free trade to continue, forging free trade deals with the ever-growing global economy will be nigh on impossible.”
Critics have suggested that Labour’s new policy is designed to bring down Theresa May’s Conservative government ahead of a key vote in the Commons soon on whether to remain in the Customs Union after Brexit.
Pro-EU Tory rebel Anna Soubry tabled an amendment that seeks to bind Britain to the customs union beyond 2019 and has received support from Ken Clarke, Nicky Morgan, Dominic Grieve and others on the government benches.
It is not clear whether the Government will have a majority when the amendment is voted on.
Mr Littlewood also responded on Twitter to Labour plans to nationalise industry: “So @jeremycorbyn argues we don’t need competition in postal delivery as we each have just one letter box. Presumably, we don’t need competition in food supply either as we each have just one mouth.”