Three Conservative MPs who resigned to join a new independent group on Wednesday said Theresa May had allowed their former party to fall prey to hardline Brexiters and declared that the Tory modernising project had been destroyed.

In the latest evidence that Brexit is reshaping the political landscape, Heidi Allen, Anna Soubry and Sarah Wollaston, all outspoken critics of May’s stance on Europe, said the Conservative party as they had known it under David Cameron was dead.

“I’m not leaving the Conservative party – it has left us,” said Soubry at a hastily convened press conference around the corner from the House of Commons. “The modernising reforms that had taken years to achieve were destroyed.”

Allen was asked if she could ever return to the Conservatives and answered: “If we do our jobs properly, there won’t be a Tory party to go back to.” She added: “We’re about creating something better that is bang smack in the centre ground of British politics that people out there, I am convinced, we are convinced, want.”

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The dramatic resignations – announced shortly before May confronted Jeremy Corbyn at prime minister’s questions – sent shockwaves through Westminster, where MPs had barely digested news of the Labour split.

The move reduces May’s already tenuous working majority to eight, raising still more questions over her authority amid rumours that there could be further Tory defections.

On Wednesday night, Allen told ITV’s Peston that “a third of the party” – around 100 of her former colleagues – shared her frustrations at its direction. The Tory former attorney general Dominic Grieve told the BBC: “The government which I am supporting implementing a no-deal Brexit - what would I do? I would not be able to maintain my support of the government. I would have to leave the party.”

The arrival of former Conservatives in the Independent Group, alongside breakaway Labour MPs including Chuka Umunna and Chris Leslie, was immediately seized on by the Labour leadership, which stepped up attacks on the defectors.

Play Video 0:52 MP Heidi Allen quits Tories after having to fight 'for benevolence' – video

“The fact is they have formed what is effectively an establishment coalition based on the failed and rejected policies of the past: austerity, corporate tax cuts, privatisation,” Corbyn’s spokesman said.

“It’s precisely because those policies were seen to have failed and were rejected that the direction under the Labour party has changed since Jeremy was elected. And we demonstrated at the general election that a different approach has mass electoral appeal.”

The shadow justice secretary, Richard Burgon, a close ally of Corbyn, compared what he called “Chuka’s coalition” with the national government formed by the Labour leader Ramsay MacDonald in 1931, which implemented “deep cuts and attacks on working-class communities”.

The Independent Group (@TheIndGroup) 'We can no longer act as bystanders. We are honour bound to put our constituents’ and country’s interests first.' Read the letter to the Prime Minister from @heidiallen75 @Anna_Soubry and @sarahwollaston #ChangePolitics pic.twitter.com/1HxHOULbft

Momentum, the grassroots campaign group set up to back Corbyn’s leadership, announced plans to unleash what it calls “mass canvassing events” in the constituencies of three of the Labour MPs – Umunna, Ann Coffey and Angela Smith – in the weeks ahead.

Laura Parker, Momentum’s national coordinator, said: “Their constituents voted for a Labour MP standing on a Labour manifesto. Now they’ve left the party and joined a Blairite-Tory coalition standing on a completely different platform.”

She said their decision to resign from Labour and sit as independents was “unfair, undemocratic and dishonest. If they care about their constituents rather than their own careers they should step down and fight a byelection.”

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However, some senior Labour figures have taken a markedly more sympathetic tone towards the departing MPs. The shadow trade secretary, Barry Gardiner, said Luciana Berger, the MP for Liverpool Wavertree who blamed antisemitism for her decision to quit Labour, had suffered “disgraceful treatment”.

“I regret deeply that she has left our party, I regret most of all the antisemitic abuse which made her feel it was necessary to do so,” he told MPs in a Commons debate on antisemitism. He described her as a “kind and loving person” who has been “bullied by antisemites to a point that most of us would not have had the strength to bear”.

The Independent Group was formed on Monday by the erstwhile Labour MPs. Joan Ryan became the eighth MP to quit Labour and join the group on Tuesday night. The MPs claimed they felt it necessary to leave Labour because of the leadership’s Brexit policy and failure to tackle antisemitism in the party.

The 11-strong parliamentary grouping formed by the breakaway MPs is not yet a formal political party but its founders hope to swell their numbers further in the coming days. Several potential defectors told the Guardian they were not ready to join the group – “yet”.

Soubry said she and her Tory colleagues had picked up the baton handed to them by Umunna and were now holding it out to other “one-nation Conservatives” dismayed by May’s failure to take a no-deal Brexit off the table.

“Dear friends, now former colleagues, who share those one-nation values and principles will deny it, but I believe in their heads and in their hearts that they know it is over,” she said, adding: “The battle is over. The other side has won.”

“The right wing, the hardline, anti-EU awkward squad that have destroyed every leader for the last 40 years are now running the Conservative party from top to tail – they are the Conservative party.”

The group has not yet published any policy proposals, just a set of values, including supporting “a diverse, mixed social market economy” and removing the barriers of “poverty, prejudice and discrimination” to ensure inequalities can be “reduced through the extension of opportunity”.

The challenges of uniting them around a joint policy position were underlined at Wednesday’s press conference when Soubry was asked about the spending cuts of the 2010-15 coalition government, in which she was a minister.

“I think the things we did to the economy were absolutely necessary at the time. I don’t have a problem with that,” she said. Leslie, who as shadow chancellor criticised the 2015 Labour manifesto as too leftwing, later said the party was keen to be forward-looking.

Wollaston pointed out that after the prime minister lost the meaningful vote on her Brexit deal by an unprecedented margin of 230 votes last month, she promised to seek a cross-party solution – but the first group she consulted was the hard Brexit European Research Group.

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In their joint resignation letter, the three MPs said: “We no longer feel we can remain in the party of a government whose policies and priorities are so firmly in the grip of the ERG and DUP. Brexit has redefined the Conservative party.”

But Allen, who represents South Cambridgeshire, said of the new grouping that she felt “so excited, in a way I haven’t felt since I was first elected … I, we, are prepared to dare to dream that this could be possible.”

The latest defections are unlikely to change the parliamentary arithmetic in next week’s crucial votes, because the 11 MPs were already confirmed Brexit rebels. But they underlined the sense that both May and Corbyn have struggled to contain the deep divisions in their parties, on Brexit and beyond.

A string of ministers are privately warning that they are prepared to vote against the government and back Yvette Cooper’s effort to force an extension to article 50, rather than allow May to continue using the threat of a no-deal Brexit as a bargaining chip.

Before flying to Brussels for talks with Jean-Claude Juncker on Wednesday, May said she was “saddened” by the decisions of Soubry, Allen and Wollaston to resign, saying they were “people who have given dedicated service to our party over many years, and I thank them for it”.

Despite the divisions over Brexit, she said: “I am determined that under my leadership the Conservative party will always offer the decent, moderate and patriotic politics that the people of this country deserve.”

The work and pensions secretary, Amber Rudd, tweeted that she was keen to continue to work with the departing Tory MPs on “a number of important issues”, including Brexit, in the future.

Soubry suggested she and her colleagues would support the prime minister in any future no-confidence vote in the government, because now was not the time for a general election. May survived a no-confidence vote with a majority of 19 last month.

Despite the extraordinary drama, neither party leader mentioned the resignations at prime minister’s questions, as the 11-strong Independent Group sat perched high up on the opposition benches.

• The caption to the graphic was amended on 21 February 2019 because an earlier version omitted to credit the Institute for Government.