Stressing that building closer economic ties was a “sovereign decision”, the prime minister said that doing so had led to the best relationship with the EU “for 10 years”.
“What I’m saying is, there are other areas where we should consider whether it’s in our interest to do the same and align with the single market,” he said.
“Now, that needs to be considered on an issue-by-issue, sector-by-sector basis, but we’ve already done it with food and agriculture and that will be implemented this year.”
Sir Keir’s comments follow pressure from within the Labour movement to go further on a customs union, with 13 backbenchers backing proposals that would pave the way for such an arrangement in a Commons vote in December.
Reminded of his promise that “Brexit is safe in my hands”, the prime minster insisted this was not an attempt to reverse it – because nobody wanted to be “picking over the bones of Brexit” but rather “looking forward” at what was in the national interest.
Elsewhere, concerns have been raised that current negotiations around a youth mobility scheme for British and EU students could end up requiring the UK to sign up to the free movement of EU citizens.
Sir Keir said: “That is not the return to freedom of movement, we’re not going back to freedom of movement.
“But I personally think that young people having that opportunity is a very good thing.
“And when we announced just recently that we’re going back to the Erasmus scheme so that young people can study, exchange, research can be done in a much better way than it is now – that was widely welcomed.”