As many as 100 MPs could be prepared to support a vote of no confidence in David Cameron if Britain votes to remain in the EU, Tory sources have claimed.
Backbenchers furious at the way Mr Cameron and his allies have conducted the referendum campaign are considering calling for him to go, even if he wins the June 23 vote by a significant margin.
Just 50 Tory MPs are needed to trigger a vote of no confidence in the leader.
Around 150 Conservative MPs are thought to back a Brexit, although a number of them have not publically stated their intentions.
Mr Cameron will almost certainly win a no confidence vote, allowing him to continue as Prime Minister.
However, the rebels will then threaten to ensure that Government legislation cannot pass in the House of Commons unless Mr Cameron pledges to step down as Prime Minister earlier than 2019.
Mr Cameron’s opponents believe that if he quits as Prime Minister before 2019, it increases the chances of his successor being a minister who backed the Brexit campaign, most likely to be Boris Johnson.
They are concerned that if Mr Cameron remains in Downing Street until just before the next general election in 2020, he will be able to spend the next four years “rehabilitating” George Osborne, whose popularity in the Tory party has nosedived in recent months.
Boris Johnson’s Vote Leave speechPlay!08:11
According to sources, MPs will in the weeks after a Remain vote call for a vote of no confidence in Mr Cameron.
The plotters are angry about Mr Cameron’s decision to spend £9million of taxpayers’ money on a pro-EU leaflet sent out to every household in the UK.
The have also been left enraged by his warnings about the consequences of a Brexit vote, particularly his suggestion that it could lead to war in Europe.
They believe that the campaign “has not been a fair fight” and will “show Cameron that it will not be forgotten by the Eurosceptics in the Conservative Party”, a source said.
Mr Cameron has a slim Parliamentary majority of 12, meaning rebels in his party can easily prevent legislation from getting through the Commons.
Ahead of the 2015 general election, Mr Cameron said that he will step down after serving a full second term in office.
Mr Cameron and his allies have insisted that he will stay on as Prime Minister “until the end of this Parliament”.
EU referendum in one word: are you Team Cameron or Team Boris?Play!01:25
However, MPs planning the vote of no confidence want to force him to set out “an earlier timetable of departure”.
TAP – The battle to leave the EU could run and run. The Referendum could be rigged, but not a Commons vote. If Johnson wins control of the Parliamentary Party with full support of Farage and UKIP’s 4 million plus supporters, the exit from the EU could be reached the Parliamentary way.