Former chief whip Mark Harper today vows to break 13 years of loyalty to the Government by voting against Theresa May’s Brexit plan, saying: “Brexit should be an opportunity for our country to spread its wings, not have them clipped.”

The respected MP, previously a May loyalist who voted remain, claims the proposed withdrawal agreement “not only compromises the integrity of our country” but also breaches the Conservative manifesto, insisting it “leaves the UK in a worse position that we are now”.

Former chief whips are usually unwaveringly loyal to the Government so the former Immigration Minister's uncharacteristic rebellion will be a blow to the Prime Minister, whom he served under at the Home Office.

Writing in the Telegraph, he says: “The Cabinet’s proposals are not acceptable because they threaten the integrity of our country, keep us trapped indefinitely in a customs union and leave us in a weak negotiating position for our future relationship.”

In a damning indictment of Mrs May’s negotiations with Brussels over the Irish backstop, he adds: “The Prime Minister said that the EU’s proposal would undermine the UK common market and threaten the constitutional integrity of the UK by creating a customs and regulatory border down the Irish Sea, and no UK Prime Minister could ever agree to it.