Former Education Secretary Justine Greening labelled Chequers "less popular than the poll tax."

Greening's intervention came as Boris Johnson also attacked the deal in his latest column at the Telegraph.

Nick Boles, seen by some to be an outrider for Michael Gove, also attacked the deal and urged May to shift towards accepting temporary membership of the European Economic Area instead.

May appears increasingly isolated amongst the parliamentary Conservative Party, who have not calmed over recess.

LONDON – Theresa May's Brexit plans have been branded "less popular than the Poll Tax" by a former minister, as the prime minister comes under increasing pressure from both pro-remain and pro-Brexit MPs abandon her so-called Chequers plan for Brexit.

The former Education Secretary and Remain-supporting Conservative MP, Justine Greening, told the BBC: "Chequers is now dead and there's no point having a government spending two vital months working on that when what it needs to do is start working on an alternative route for us...The biggest problem for me is how people see this. The Chequers deal is now more unpopular with the British people than the poll tax was, and that is why it’s untenable to take forward."

Polling suggests that as few as 12% of voters back the Chequers proposals agreed by May and her cabinet earlier this year. While hugely unpopular, Margaret Thatcher's Community Charge, known as the Poll Tax, still retained the support of around one-in-three voters across the UK according to Gallup polling at the time.

The plan, which would mean Britain staying in a single market for goods with the EU, has so far been criticised by both wings of the Conservative party, the Labour party and EU negotiators.

Meanwhile a new poll of voters in Conservative marginal constituencies conducted by a polling company owned by former Conservative party election chief Lynton Crosby, found over half opposed to May's plans.

Crosby is suspected by Downing Street of being behind a plan to oust May and replace her with her former Foreign Secretary Boris Johnson.

Boris Johnson PA

Johnson on the attack

On Monday Johnson published a comment piece in the Daily Telegraph attacking May's Brexit strategy and accusing her of waving the flag of surrender to the EU.

Johnson said: "In adopting the Chequers proposals, we have gone into battle with the white flag fluttering over our leading tank. If we continue on this basis we will throw away most of the advantages of Brexit.

"The fix is in. The whole thing is about as pre-ordained as a bout between Giant Haystacks and Big Daddy; and in this case, I am afraid, the inevitable outcome is a victory for the EU, with the UK lying flat on the canvas and 12 stars circling symbolically over our semi-conscious head.

Johnson went on to argue that "some" ministers used the Irish border as a political tool to "stop a proper Brexit."

Nick Boles Grantham Conservatives

An alternative proposal

Downing Street was quick on Monday to point to Johnson's inability to spell out any alternatives to her proposals. However, Senior Conservative backbencher Nick Boles has argued for an alternative plan.

Boles outlined his vision, seeking to cancel any divorce payments to the EU, prevent an Irish border separating the UK from Northern Ireland, and ending any UK affiliation with Common Agricultural or Fisheries policy.

Boles called for the UK to stay within the European Economic Area (EEA) for an interim basis, while trying to strike a Free Trade Agreement (FTA) with the EU. If that fails, Boles would also like to see proper preparation for WTO terms.

Boles was a founding member of the Brexit Delivery Group, which was a grouping of around 20 Conservative MPs, which sought to "give the Prime Minister and Government the peace to operate" and was touted for a post-recess rollout.