Former chief whip Andrew Mitchell said he was amazed party performed as well as it did in local elections

A senior Tory has said he is surprised that anyone bothered to vote Conservative in last week’s local elections, amid growing pressure from all levels of the party on Theresa May to announce her departure.

With an air of despair descending on many MPs after a drubbing in the polls last week, Andrew Mitchell, the former chief whip, said the chaos in his party meant he had been amazed that it got the votes it did.

“I am so sorry at the loss of many excellent Tory councillors,” he told the Observer. “But frankly, given the performance of the government, it’s pretty amazing that anyone turned out to vote Conservative at all.”

A grassroots movement to force out the prime minister is gaining ground in the wake of a set of election results that saw the Tories lose an astonishing 1,300 councillors. MPs and local constituency chiefs also warned May that agreeing a Brexit deal with Labour would lead to the party’s annihilation.

Another former cabinet minister said they now regarded May as the “worst PM in living memory”, but that removing her now could precipitate a disastrous general election.

Cross-party talks between the Tories and Labour begin again in earnest this week, with Labour’s demand for a permanent customs union with the EU still a major stumbling block. However, senior Tories believe that there is a chance that May will simply decide to quit should European elections later this month prove as disastrous as many fear.

Some Tory MPs already believe that any deal with Corbyn could be dead on arrival, with swaths of the party vowing to vote it down.

“Let’s say you get half or two-thirds of the Labour party supporting it,” said one senior Tory. “You may lose two-thirds of the Conservative party. You may still get something through, but you would unleash a nightmare.”

There is also frustration among some Tories that May sacked defence secretary Gavin Williamson shortly before the local elections, after accusing him of leaking sensitive information from a meeting of her national security council. On Saturday the Metropolitan police said the leak, about Chinese tech giant Huawei, would not amount to a criminal offence as the details did not “contain information that would breach the Official Secrets Act”.

Williamson said that if the police weren’t going to investigate, a “full and impartial investigation” should be opened into “this shabby and discredited witch-hunt that has been so badly mishandled by both the prime minister and [cabinet secretary] Mark Sedwill.”

The report into the leak has not been released, but Tories are examining ways to force its publication. One of Williamson’s supporters said: “When No 10 is pressured to release the report and refuses to do so, it will become obvious who is telling the truth here.”

May is facing the extraordinary prospect of an emergency meeting of the party’s national convention, made up of grassroots members, set to be called to hold a vote of no confidence in her leadership. The gathering is expected to be delayed until after Donald Trump’s visit in June to commemorate D-Day’s 75th anniversary.

May will be pressed to give more details of her departure plan before a meeting of the 1922 Committee on Wednesday. One committee source said: “She can give us a timetable, or she can tell us to piss off. Either would be a significant moment.”

Former Tory leader and leading Brexiter Iain Duncan Smith has called for May to set out her departure plans. He said she was now a “caretaker prime minister”.

He told LBC radio: “The [1922] committee has to sit again now, urgently, and decide that either the prime minister sets the immediate date for departure or, I’m afraid, they must do it for her.”

Grassroots hostility to a cross-party Brexit deal was growing on Saturday , with activists worrying about European elections set to feature Nigel Farage’s Brexit party for the first time. Alanna Vine, chair of Cheadle Conservative Association, said: “Our party’s failure to deliver Brexit has been toxic. If we don’t change course, immediately cease discussions with Corbyn about us remaining in the EU customs union and stop endlessly extending our leaving date, our party will be wiped out for a generation.”

Alexander Curtis, chair of Hertford and Stortford Conservative Association, said: “Colleagues of mine have paid dearly for our prime minister’s failure to believe in and back the decision of 17.4 million voters to leave the EU.

“People are sick of our incompetence and inability to deliver and to honour our promises. We will be annihilated in the Euro elections if we break another promise and adopt Corbyn’s customs union plan.”