THERESA May is preparing to water down her promise to end Freedom of Movement and it will cost her the keys to No10, an ex-Brexit Department insider warns today.

David Davis’s former Chief of Staff Stewart Jackson tells The Sun that a u-turn on immigration from Europe after Brexit is set to be the “next big climb down” from Downing Street.

He describes an offer to Brussels that will keep keep borders open to EU citizens as the “last major red line” yet to be sacrificed in pursuit of a deal.

Mr Jackson quit Whitehall when David Davis resigned in the wake of Theresa May’s Brexit negotiations deal hammered out at her Chequers country house in July.

Writing for The Sun today he says: “we can see clues of further surrender in the fuzzy language from the Chequers agreement.” And the former MP warns Tory colleagues that “such a move would be politically toxic and very poorly timed” and could cost them the next election.

In a searing attack on Downing Street, Mr Jackson writes: “One of the major drivers of the Leave campaign was have: Who comes to our country and on what basis – decided by our own Parliament.”

He says that should Mrs May forget that: “It would also betray her own instincts, which she rightly gave voice to when she became Prime Minister.

Pointing to the PM's surge in popularity in 2016, he said a hardline on immigration had “struck a chord with millions of people angry and alienated at their voices being ignored by the political elite” and “delivered for her record poll ratings then and a real feeling of hope that at last people in power were listening.” And adds that to to go back on that would be fatal for her government at the ballot box.

The siren call comes as Jeremy Hunt risked fresh wrath from from Tory colleagues by warning a No Deal Brexit would cause a rupture that would “take a generation to heal” just hours before Ministers set out their plans avoid disaster.

Mr Hunt will use a speech today in the US to double down on his claims that “he risk of a messy divorce, as opposed to the friendship we seek, would be a fissure in relations between European allies that would take a generation to heal - a geostrategic error for Europe at an extremely vulnerable time in our history.”

But Brexit Secretary Dominic Raab, who today travel to Brussels for exit talks, says a No Deal option must be taken seriously.

Ahead of publication of a slew of No Deal scenario plans, Mr Raab said that a while a deal was "by far the most likely outcome" of the negotiations, a responsible Government needed to set out the steps it was taking to mitigate the risks.

EU Commission spokesman Alexander Winterstein said Brussels officials were working "at full speed, 24/7" to reach an exit agreement.

But France’s EU Commissioner Pierre Moscovici said there would "not necessarily" be an agreement between the UK and EU by next March.

The No Deal scenario plans include an offer to Brussels of a “unilateral deal” to allow EU citizens to stay and work in Britain in an event of a No Deal Brexit, even if such an offer was not returned by the EU27 for Brits living abroad.