In a message to her political rivals, the Prime Minister stressed that Brexit was not an exercise in “political theory”, but affected people’s lives and livelihoods.

There was uncertainty last night over the extent of the revolt against Mrs May’s leadership in the Tory ranks. So far, at least 26 MPs have publicly confirmed they have sent letters calling for a confidence vote to Sir Graham Brady, chairman of the backbench 1922 Committee, but still short of the 48 required to trigger a ballot.

A delegation from the European Research Group of Eurosceptic Tory MPs, including Iain Duncan Smith and Owen Paterson, met in Downing Street yesterday in a last-ditch attempt to change the goverment’s backstop plan for the Irish border.

Picture: Matt Dunham / POOL / AFP/Getty Images)

Another detail to be resolved ahead of an EU summit on Sunday is how long the transition period could be extended.

There has been speculation that a one-off extension, if sought when the agreed transition period expires at the end of 2020, could last until the end of 2022.

But Mrs May said she wanted it to be over by the next general election in June 2022, while EU chief Brexit negotiator Michel Barnier said an extension could not be “indefinite” and a deadline would be decided this week.

The Prime Minister told the CBI conference in London: “What we have agreed unashamedly puts our future economic success, and the livelihoods of working families up and down this country, first.”

But Brexit-supporting businessman Roger Kendrick challenged the Prime Minister, telling Mrs May: “Think again about the economics of the whole thing.”

In Brussels, Mr Barnier updated the other 27 EU members on the process. Austrian EU minister Gernot Blumel said it was “a painful week in European politics ... we have the divorce papers on the table, 45 years of difficult marriage are coming to an end”.

Theresa May fights corner in vital last week