Boris Johnson’s shrinking options have narrowed further on Friday, after opposition leaders agreed to reject his demand for a snap general election, until a Brexit delay has been secured.

The prime minister reportedly wrote to Tory members on Friday evening pledging to break the law that will require him to seek an extension of article 50. “They just passed a law that would force me to beg Brussels for an extension to the Brexit deadline. This is something I will never do.”

Earlier on Friday he said he would not entertain seeking another deadline extension from Brussels, as the incoming law, expected to receive royal assent on Monday, compels him to if no agreement is in place by 19 October. “I will not. I don’t want a delay,” Johnson said.

The BBC reported on Saturday that cross-party MPs, including expelled Conservatives, had sought legal advice and were preparing to go to court “to compel Mr Johnson to seek a delay”.

David Lidington, the de facto deputy prime minister when Theresa May was in Downing Street, said it would set a “dangerous precedent” if Johnson chose to disobey the law.

Lidington, a former Europe minister, told BBC Radio 4’s Today programme: “It is such a fundamental principle that we are governed by the rule of law that I hope no party would question it. Defying any particular law sets a really dangerous precedent.”

Lidington said Johnson had convinced him that he still wanted to strike a deal with the EU.

The former Conservative leader Iain Duncan Smith encouraged Johnson to break the law, saying he would be seen as a Brexit “martyr” if judges opted to put him jail for breaching parliament’s terms.

With parliament due to be suspended by next Thursday at the latest, it now appears unlikely Johnson will succeed in his bid to force an election before 31 October – unless he takes the nuclear option of resigning.

After a tumultuous week for the prime minister, the rebels’ pact came as:

• Former justice secretary David Gauke warned that Johnson risked turning the Conservative party into “Farage-lite”.

• The backbench bill aimed at blocking a no-deal Brexit completed its passage through the House of Lords, clearing the way for it to receive royal assent on Monday.

• A scrawled note leaked to Sky News showed Johnson referring to David Cameron as a “girly swot”.

• Johnson was criticised by the chief constable of West Yorkshire police for using new police recruits as the backdrop for a political speech.

• The prime minister’s first concrete proposal for replacing the Irish backstop hit the buffers following a “disastrous” meeting between his chief negotiator and the EU in Brussels.

Plaid Cymru’s Westminster leader, Liz Saville Roberts, who was involved in the rebel talks, said: “We need to make sure that we get past 31 October, and an extension to article 50. We were in agreement that the prime minister is on the run. Boris is broken. We have an opportunity to bring down Boris, to break Boris, and to bring down Brexit – and we must take that.”

She added: “Our intention is to be here, in this place, to hold him to account and to make sure that he abides by that law.”

The brutal treatment of the Tory rebels, and the erratic behaviour of Johnson’s adviser Dominic Cummings, have provoked growing disquiet even in cabinet, with Michael Gove raising the question this week of whether they could be welcomed back into the party.

Gove, who as chancellor of the duchy of Lancaster is in charge of accelerating no-deal planning, asked officials to leave the regular gathering of the key “XO” committee of ministers on Friday so they could air their concerns, one source told the Guardian – although allies insisted the meeting was routine.

Johnson will fly back on Saturday morning from a visit to the Queen’s summer residence of Balmoral with his partner Carrie Symonds to spend the weekend plotting his next move.

The passage of Hilary Benn’s backbench bill means Johnson will be legally obliged to request an extension to article 50, if he has failed to pass a Brexit deal, or received MPs’ approval to leave without a deal, by 19 October.

Speaking in Scotland on Friday, the prime minister insisted he would refuse to ask for any delay.

“We’ve spent a long time trying to sort of fudge this thing and I think the British public really want us to get out. They don’t want more dither and delay,” he said during a visit on Friday to a farm near Banchory, Aberdeenshire, at which he encountered a prize bull called Keene.

Asked how he would secure a new deal at the EU summit on 17 October, he said: “By powers of persuasion. Because there’s absolutely no doubt we should come out … It’s a pointless delay.”

But Gauke, one of the 21 Conservative rebels who lost their party whip this week for supporting the Benn bill, said Johnson would ultimately be left with little choice.

“During both the leadership election and subsequently, he has just boxed himself in, again and again and again. Just this week he is now saying there are no circumstances in which he will seek an extension. But if the law requires him to seek an extension, he either has to comply with the law, or resign. Surely he must comply with the law?”

In an interview with the Guardian, Gauke also strongly criticised the Tory party’s direction under Johnson.

If Johnson did resign, the Queen would be expected to ask Corbyn to try to form a majority government.

The Labour leader has mooted the idea of leading a short-term caretaker government, in order to extend article 50 and call a general election. But many members of the rebel alliance have concerns about backing a Corbyn-led administration, even for a short period.

There was further embarrassment after it emerged that Johnson referred to Cameron as a “girly swot” in a recent cabinet paper. The leak of an unredacted version of court documents to Sky News prompted condemnation of the prime minister for sexist insults.

During his inaugural prime minister’s questions on Wednesday, Johnson seemed to call Corbyn a “great big girl’s blouse” in relation to the Labour leader’s refusal to back an immediate general election.

The other reference dates back to 16 August, appearing in a handwritten note about the idea of suspending parliament for five weeks.

The document was initially revealed on Thursday by Downing Street as it resisted legal challenges in Edinburgh and London to the prorogation of parliament, both of which cases were eventually won by No 10.

In Aberdeenshire on Friday, at the end of a bruising week, Johnson was asked twice whether he would sack his chief strategist, Dominic Cummings, after the former prime minister, Sir John Major, described him as a “poison” at the heart of government on Thursday night.

Johnson did not answer the first question on his adviser’s future but pressed a second time refused to give Cummings his unambiguous support. He answered: “Look … advisers, as I think someone said in the Commons the other day, advisers advise and ministers decide.”

He was also asked about his own future after a tumultuous week during which he suspended 21 Tory MPs, including the former chancellor Ken Clarke, his younger brother Jo Johnson stepped down from the government and as an MP, and other senior Tories announced their retirement. His brother cited irreconcilable conflicts between his family and the national interest.

Asked about his failure to uphold his pledge during the Tory leadership campaign to unite the party, Johnson said he had promised to “deliver Brexit, unite the country and defeat Jeremy Corbyn. And that’s what we’re going to do.” Asked again when he would resign, he said: “Er … well … I think after those three objectives have been accomplished I will … At some point after those three objectives have been accomplished.”

With a no-deal Brexit now all but impossible, Johnson will come under intense pressure to achieve the renegotiated deal with Brussels he has always said was his preferred option.

To encourage him to do so, a new cross-party group launched on Friday called “MPs for a deal”, including Labour’s Stephen Kinnock and Caroline Flint, Liberal Democrat Norman Lamb, and former Conservative leadership contender Rory Stewart, who lost the party whip this week.

“Even at the eleventh hour it’s not too late to agree a deal to ensure an orderly exit from the European Union,” they said in a joint statement. “The media focus has been on the short extension proposed by the Benn bill. However, we believe that Boris Johnson’s response to the bill should be to ensure he secures a deal with the EU27.”