Theresa May's chief Brexit adviser secretly warned her that the customs backstop is a "bad outcome" for the UK which will see regulatory checks in the Irish Sea and put security co-operation at risk, The Telegraph can reveal.

Oliver Robbins warned in a letter to the Prime Minister that there is no legal "guarantee" that Britain will be able to break off from the backstop, potentially leaving the UK trapped in a Customs Union with the EU.

He argued that extending the transition period after Brexit would provide a more "cast iron escape route" than entering into the backstop, which under Mrs May's deal will kick in if a solution to the Irish border issue cannot be found by December 2020.

He said: "We should not forget that the backstop world, even with a UK-EU customs union, is a bad outcome with regulatory controls needed somewhere between GB and NI, serious and visible frictions and process between GB and the EU, and no security co-operation provided for."