"How has the Labour position shifted?
Labour’s 2017 manifesto said merely that the party wanted to retain “the benefits of the single market and the customs union”, and did not say the UK should stay in either. In recent months, however, a series of senior Labour figures have argued for the UK to be in “a” customs union post-Brexit.
What’s the difference between “a” customs union and “the” customs union?
Labour says the latter is the existing arrangement, which ends when we leave the EU, and that “a” union could retain the bulk of the benefits without overly tying the UK to rules made in Brussels. Critics, mainly in the government, argue that this could be seen as Labour’s own version of an unrealistic “cake and eat it” approach.
Why has Labour's position moved?
Corbyn has never seemed that keen on the customs union, but he has faced pressure both from members of his team – the shadow Brexit secretary Keir Starmer has played a key role – and the fact that the majority of Labour members support customs union and single market membership. There is also the incentive that Labour could defeat the government in the Commons by voting with Tory rebels on an amendment to back the customs union."
From the Guardian.