Tory MPs will be ready in sufficient numbers to force Boris Johnson out of Downing Street within weeks if he tries to dodge responsibility for rule-breaking parties at No 10, the Observer has been told.

While most Conservative MPs say they are waiting for a report into so-called “partygate” by the senior civil servant Sue Gray before deciding the prime minister’s fate, large numbers admit privately that their minds are effectively made up and that they are merely observing “due process”.

What has convinced many Tory MPs to act against Johnson if he or the Downing Street operation is criticised directly or indirectly by Gray – and he then tries to fight on – has been the furious reaction from their constituents to new revelations of partying, including two events late into the night at No 10, on the eve of Prince Philip’s funeral last April when such gatherings were forbidden.

An Opinium poll for the Observer will spread further alarm in Conservative ranks.

It shows Labour taking a 10-point lead over the Tories, with Johnson’s personal ratings collapsing to levels equivalent to Theresa May at her lowest ebb. It also shows that 46% of Leave voters who backed the Conservatives at the 2019 election say Johnson should resign, suggesting the coalition he put together to win an 80-strong majority in parliament is fracturing.

The Tory MP Tobias Ellwood, a former foreign office minister, said that Johnson had to demonstrate within days that he could end the chaos for good and demonstrate a new kind of leadership. “But if he tries to spin his way out of a critical report, he will lose my support,” he said.

Tim Loughton, Conservative MP for East Worthing and Shoreham wrote on Twitter: “I have regretfully come to the conclusion that Boris Johnson’s position is now untenable, that his resignation is the only way to bring this whole unfortunate episode to an end and I am working with colleagues to impress that view on Number 10.”

So far only a handful of Tory MPs have called publicly for Johnson to go. To trigger a vote of confidence in him, 54 MPs or more need to write to the chair of the influential 1922 Committee of Tory backbenchers asking for one. If Johnson were to lose such a vote, he has to step down.

Another senior figure in the party said MPs were in a holding pattern. The Tory MP for Waveney in Suffolk, Peter Aldous, said he was close to calling for Johnson’s head though he would wait for the Gray report, adding that many pro-Brexit voters in his constituency were among those who were outraged at what had been going on.

“An awful lot people who voted Brexit do feel very badly let down,” he said, pointing out that this was not because of the effects of Brexit but because of Johnson’s personal behaviour.

Another Tory MP said that he had been confronted by local Tory councillors on Friday who had voted for Johnson as leader and backed Brexit before voting Conservative in 2019, but were now insisting en masse that Johnson had to go. “I can say my councillors are very pissed off,” he said.

A demonstrator protesting about ‘partygate’ in London last month.

A senior Whitehall figure in regular touch with Gray, who is expected to publish her findings by the end of this week, said she was “disappointed in what she is finding but personally she is robust”. It is understood she had no idea of the latest revelations about parties and only found about them in the media.

The source added that Gray had her reputation for independence and integrity to protect but was also studiously fair. “She will give it both barrels to whoever deserves it, whether that is a special adviser or civil servant or the politicos.

“We will get the unvarnished truth. If she sees direct responsibility, she will be clear. That is why I think this is going to be uncomfortable for everyone. Everyone is going to take their share of the blame.”

Professor Robert Ford of Manchester University said the Opinium poll should be seriously worrying for the Tories and Johnson. “Johnson himself has shifted from the party’s biggest asset, with a Brexit-fuelled appeal separate to the Conservative brand, to its biggest liability, as the revelations of Downing Street revelry in lockdown render him personally toxic with the same voters.

“The Conservatives may now be damned if they do and damned if they don’t. Sticking with Johnson risks further contaminating the party brand if the scandals continue and voter fury intensifies.”

Separately, a panel of Tory activists assembled by the ConservativeHome website found 53% believed Johnson should resign immediately. The former Tory MP and editor of the website, Paul Goodman, said: “That over half of party members want the prime minister out, if this return is representative, is a humiliating result for him, and shows a loss of confidence that may not be reversed.”

Opinium put Labour on 41%, the Tories on 31%, the Lib Dems on 9% and the Greens on 6%.

On the partygate scandal, 78% of voters think that Downing Street staff did break lockdown rules, against 8% who say they did not. Some 76% think Johnson himself broke lockdown rules, against 8% who believe he did not. Just 13% think Johnson is telling the truth about what happened, against 64% who do not. Some 67% believe the police should investigate what happened.