The former Tory chancellor Philip Hammond held private talks with Labour’s Brexit spokesman Keir Starmer shortly before Boris Johnson entered Downing Street last Wednesday, to plot cross-party moves aimed at preventing the new prime minister agreeing to a no-deal Brexit.

The meeting in the House of Commons – which took place shortly after Hammond had resigned from the government – is evidence of the fierce backlash Johnson faces from MPs of all parties if he tries to defy parliament and take the UK out of the EU without an agreement on 31 October.

It is understood that the former political opponents Hammond and Starmer agreed to work together through the summer recess with other leading parliamentarians who oppose no deal, including former Tory ministers Oliver Letwin and Dominic Grieve, to thrash out how best to use parliamentary votes to torpedo no deal.

On Saturday night Starmer confirmed that Johnson’s arrival in No 10 had spurred more cross-party discussions at high levels involving senior Tories sacked by Johnson, or who chose to resign, as opponents of no deal prepared a cross-party counter-offensive against his new hard-Brexit cabinet and government.

“The political direction of travel under Boris Johnson is clear,” said Starmer, “and so it is more important than ever that we build a strong cross-party alliance to stop a no-deal Brexit.

“That work will intensify over the summer, before parliament resumes in September.”

The plans being hatched include amending Brexit-related legislation that has to pass through parliament before the UK can leave the EU in a way that would force the Johnson to ask for a further extension to the UK’s membership if no Brexit agreement has been reached by early October.

A “last resort” option is for Hammond and other Tory Remainers to vote for a no-confidence motion in their own government if no deal still appears on the cards.

A new Opinium/Observer poll on Sunday will heighten fears among Remainers that Johnson could resort to calling a snap Brexit election to seek a mandate for a no-deal Brexit if he fails to persuade EU leaders to reopen talks on the Irish backstop and Theresa May’s withdrawal agreement.

The poll, conducted after Johnson was elected Tory leader, shows a marked “Boris bounce”, putting the Tories up seven points compared with a fortnight ago, on 30%.

The Conservatives have leapfrogged Labour (which is 3 points up on 28%) to retake the lead. The surge in support appears to come at the expense of the Brexit party which has dropped 7 points to 15%.

The Liberal Democrats, whose new leader Jo Swinson took office last week, are up 1 point on 16%.

Johnson also has a 21-point lead over Jeremy Corbyn when voters are asked who would make the best prime minister. It is the biggest lead since May forged ahead of Corbyn before the 2017 election. Half of the public think, however, that Johnson will divide the nation.

Professor Robert Ford of Manchester University said that Johnson’s pursuit of Labour voters who backed Brexit could win the Tories some seats but could also cause losses for the party in Remain areas, where voters would reject his talk of no-deal Brexit.

“ A dramatic gamble may be needed to break the Brexit deadlock, but such gambles can easily backfire, as we saw in 2017,” Ford writes.

On Saturday Johnson further fuelled speculation that he was preparing for an election on a visit to Manchester. In a speech that bore all the hallmarks of an election pitch to disillusioned Leavers in Labour areas, he pledged a £3.6bn fund for deprived towns, a new Manchester-to-Leeds rail line and major investment in buses, broadband and police.

Johnson pledged a £3.6bn fund for deprived towns and a new Manchester-to-Leeds rail line while in Manchester on Saturday. Photograph: Getty Images

“The centre of Manchester, like the centre of London, is a wonder of the world. A few miles away from here, the story is very different,” he said.

“The story has been for young people growing up there of hopelessness, or the hope that one day they will get out and never come back.

“It certainly isn’t really the fault of the places, and certainly isn’t the fault of the people growing up there.

They haven’t failed. It’s we, us, the politicians, our politics has failed them.” Officially Johnson has ruled out a general election before Brexit has been delivered, but many MPs believe he will be prepared to switch to backing one if he cannot secure a deal on Brexit that he can sell to his own party and get through parliament.

Meanwhile Corbyn, who is having to step up election preparations, on Sunday will accuse the Tories of causing a £40bn of lost investment in 2019 as a result of the Brexit impasse.

Labour claims that it has reached the figure by comparing forecasts by the Office for Budget Responsibility before and after Brexit. It said businesses are expected to invest around £187bn this year, compared to the £227bn forecast in 2016.

“Boris Johnson won the support of fewer than 100,000 Tory party members by threatening a reckless no-deal Brexit, leaving businesses and workers facing serious risks and huge uncertainty,” Corbyn said. “He is staking all our futures on a sweetheart trade deal with Donald Trump that would risk the takeover of our NHS by US corporations, while handing out tax cuts to the richest.

“With Johnson and his divisive hard-right cabinet gambling with people’s jobs and living standards, it is now clear that the only way Brexit can be resolved is by taking it back to the people.”