Members of the European Research Group (ERG) threatened to vote down Government legislation last night, after claims the Prime Minister will use Labour MPs to push her Chequers plan through the Commons.
At the annual Conservative Party Conference last week, all 18 cabinet ministers gave their backing for May to reach a deal and MPs would then up approving it.
Yesterday however, Veteran Eurosceptic Bernard Jenkin told a WhatsApp group of Tory MPs if Mrs May failed to deliver an EU deal they saw acceptable, Brexiteers could defy No.10 on other key votes.
He wrote: “Make no mistake.
“A soft/non-Brexit pushed by the Conservative establishment but put through with Labour support will look like we are abandoning our supporters and remove any sense of obligation among Conservative Brexit-supporting MPs to continue to support the Government.”
Mrs May’s Brexit plan would see the UK remain in a customs union with the rest of the EU for a prolonged period.
But with the rebels plan to vote down budget motions this month, Mr Jenkin said Mrs May’s idea would “destroy party unity”.
One rebel said: “She hasn’t got a majority and, by God, she’s going to be shown she hasn’t got a majority.”
The MP for Mid Bedfordshire Nadine Dorries criticised the deal, while Simon Clarke, MP for Middlesbrough South, said it would jeopardise the Tories marginal seats at the next election, predominantly in the north of England and the Midlands.
Mr Clarke said: “I don’t know how we argue that the Conservatives delivered Brexit if we go down this route.”
Leading members of the ERG said they would support proposals that could lead EU officials working in UK ports after Brexit.
The disagreement surfaced after the international development secretary, Penny Mordaunt, was prohibited from No.10.
It is believed she was banned from briefing extracts of a speech she plans to make about Brexit on Tuesday because she did not plan to praise May’s current Chequers plan for Brexit.
Ministers have previously publicly backed Mrs May’s plan but have struck a more cautious tone recently after the plans were rejected by the EU.
Last month Michael Gove said the Chequers deal did not have to be the permanent relationship the UK has with the EU.