Attempting to rejoin an EU customs union would be “complete fuel for Reform”, Sir Keir Starmer has been warned.

Labour peer Harriet Harman told Beth Rigby on the Electoral Dysfunction podcast she thinks the government may not rejoin the current customs union, but will be part of elements of it.

On Tuesday, 13 Labour MPs backed a Liberal Democrat bill calling on the government to begin negotiations on joining a bespoke customs union with the EU.

Sir Keir recently said such a move was “not currently our policy”, but the vote, which has gone through to a second reading, has prompted suggestions the UK could join a new personalised trading partnership with the EU.

Baroness Harman said: “In the run-up to the election, Keir Starmer made it quite clear that, if he was in government, he would not be rejoining the single market or rejoining the customs union.”

But she said he could get around that by doing something similar to what the chancellor did with income tax at the budget.

Rachel Reeves extended the freeze on income tax, allowing her to stick to her manifesto promise not to raise income tax, while pulling more people into paying higher rates of the tax.

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Baroness Harman said: “I think we might get into a little bit of that with the customs union, because we’re not rejoining the customs union, but we could have a customs union which is better – i.e. elements of it, which smooth the path of better trading relations.”

But Ruth Davidson, the former Scottish Conservative leader, warned that could cause “some political problems for Labour”.

“This is the sort of thing that completely arms Reform,” she said.

“And it would take months, into years to get done.

“It would just be complete fuel for Reform.

“I think, politically, it’s too hard to do, even if economically it looks like a lever that could be beneficial to the country.”