Theresa May is attempting to placate a growing rebellion over her plans for Brexit as 63 Tory Eurosceptics issued a major new challenge to her authority and a Government aide said her proposals could cause Leave voters to “lose faith in our democracy”.

The Prime Minister’s advisers are attempting to craft new language for the Brexit withdrawal agreement amid signs the Cabinet will otherwise refuse to agree to her proposals for a “backstop” plan that would “temporarily” keep the UK in the EU’s customs union.

On Saturday night Brexiteer MPs sought to dispel claims by allies of Mrs May that they could be railroaded in the Commons with the help of rebel Labour votes.

In a highly unusual show of strength, a letter attacking the Government over its Brexit forecasts was signed by 63 Conservative MPs, including David Davis, the former Brexit secretary, Jacob Rees-Mogg, the chairman of the European Research Group of Eurosceptic backbenchers, and Steve Baker, the former Brexit minister.

Separately, writing in The Sunday Telegraph, Anne-Marie Trevelyan, a pro-Leave MP who is parliamentary private secretary at the Department for Education, said remaining in a “temporary customs arrangement” after the end of the transition period in December 2020 “means simply delaying Brexit and causing the 17.4 million people who voted for it to lose faith in our democracy, and in our democratic and legal institutions”.