Emmanuel Macron will warn Theresa May that Britain will never have a say over trade negotiations if it joins a customs union with the EU at tonight’s summit in Brussels, in a blow to the prime minister's hopes of securing a cross-party Brexit compromise.

Mr Macron will insist that whatever the result of cross-party talks on the future UK-EU relationship, the “autonomy of EU decision-making” must be protected, including on trade.

Whatever compromise is found, Mr Macron will warn, it must respect the EU’s long-held Brexit red lines and will not allow any “cherry-picking”.

An EU diplomat said, “If Britain relaxes its Brexit red lines, we can help but we are not changing any of our red lines.”

“We are not talking about another Chequers,” another EU diplomat said, referring to the prime minister’s rejected Brexit plan, which only followed some of the EU’s single market rules.