Theresa May
Prime Minister Theresa May urged by Brexiteers to deliver a “clean Brexit” Reuters

Theresa May is reportedly facing a coup plot if she attempts to keep Britain in a customs union with the EU, Conservative MPs have warned.

The reported plan would see eurosceptics install Boris Johnson as prime minister, environment secretary Michael Gove as deputy prime minister and Jacob Rees-Mogg as chancellor.

Johnson is said to have told one plotter he would be "ready" for a future contest, the Sunday Times reported, while urging MPs to "rally round" the prime minister.

The news comes amid Tory clashes over whether Britain should remain in a customs union with the EU after leaving the bloc.

Meanwhile, Tory backbencher and Leave supporter Bernard Jenkin has urged Theresa May to deliver a "clean Brexit", amid concerns chancellor Philip Hammond is pushing for a "softer" exit from the EU.

Jenkin wrote in the Sunday Telegraph: "She can only command a majority in Parliament on her present policy. Nearly half David Cameron's MPs, voted Leave, despite his patronage and pleadings. There would have been few Remain Tories if he had advocated Leave.

"Her MPs will back her, because we are overwhelmingly at one with the majority of the British people who now want a clean Brexit and an end to the present uncertainty.

"It is time for all her ministers to back her too and to end the confusion they are fomenting in government."

Speaking to the Observer, Lord Andrew Turnbull, who was cabinet secretary under Tony Blair, likened attacks by Brexiteers on the civil service to tactics by rightwing nationalists in 1930s Germany.

He said the attacks reminded him of the "stab-in-the-back" myth that emerged prior to the Second World War.

"'Dolchstoss' means 'stab in the back'," he said. "After the first world war there was an armistice, but the German army was then treated as the losers. Then, at the start of the Nazi era, the 'stab-in-the back' theme developed.

"It argued that 'our great army was never defeated, but it was stabbed in the back by the civilians, liberals, communists, socialists and Jews'. This is what I think these critics are trying to do. They are losing the argument in the sense that they are unable to make their extravagant promises stack up, and so they turn and say: 'Things would be OK if the civil service weren't obstructing us'.

"When you don't succeed, you find someone to blame for your failure."