A group of 45 American senators have signed a letter to Boris Johnson pledging to back a trade deal with Britain whether or not the UK leaves the EU with a deal. In an unprecedented show of support for the new prime minister, the Republican senators say they will come to Britain’s aid if Johnson is forced to leave with no deal.

They also vow to maintain full co-operation on intelligence through the “five eyes” alliance and Nato.

The letter, which was put together by Senator Tom Cotton from Arkansas, congratulates Johnson on his election as leader and openly compares him to his hero Winston Churchill, urging him to “roar” for Britain as Churchill did during the Second World War.

The senate intervention is a