Keir Starmer has warned that Britain must be braced for shared institutions such as supranational courts if it wants a “comprehensive” relationship with the EU.

Criticising Theresa May's Florence speech at a Scottish Fabians event, the shadow Brexit secretary said the Prime Minister's approach was inconsistent. He also repeated his call for the customs union to be "on the table" in a final deal.

He said: “If we want a comprehensive deep relationship with the EU going forward - and the answer to that is yes we do we absolutely do - then that comes at a price of shared institutions.

“You can't have a deep relationship even on trade without shared institutions.”

In her speech, May acknowledged that trade and regulatory disputes would need to be brokered by a third party, but said this could be neither the European Court of Justice or the British courts.

Starmer warned that the alternative to a court would be the kind of much-maligned investor courts proposed in the US-EU Transatlantic Trade and Investment Partnership deal.

He said: “Even the Canada-EU deal, which is a lot less than the single market, in the end has yielded to the idea of a supranational court because they couldn't do the deal without it.”

Discussing the single market and the customs union, he said that "the form follows the function" and "let's not squabble about the model at the outset".

When it came to a final deal, Starmer said: "We should be flexible about a changed relationship with the single market if that's what it takes, or something new, a bespoke agreement.

"As far as the customs union is concerned, it should be on the table as a viable option - staying in a customs union."