Joan Ryan has become the eighth Labour MP to resign and join the breakaway Independent Group, claiming Jeremy Corbyn’s party has become “infected with the scourge of anti-Jewish racism”.

Ryan, the MP for Enfield North, said she had been a member for four decades but could no longer remain as a Labour MP.

Echoing Luciana Berger, the Jewish MP for Liverpool Wavertree, Ryan blamed what she claimed was the Labour leadership’s “dereliction of duty” in the face of the “evil” of antisemitism, for her decision to resign.

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In a stinging resignation letter, she said: “I cannot remain a member of the Labour party while this requires me to suggest that I believe Jeremy Corbyn – a man who has presided over the culture of anti-Jewish racism and hatred of Israel that now afflicts my former party – is fit to be prime minister of this country. He is not.”

Ryan, who lost a non-binding no confidence vote by her constituency Labour party in September, will join the seven MPs who stepped down on Monday, and sit as an independent in the House of Commons. Her departure will stoke fears that Labour could be hit by rolling resignations in the days and weeks ahead.

Ryan told BBC Radio 4’s Today programme she had left because: “I can no longer ask people to vote for Jeremy Corbyn to be prime minister of this country because I don’t believe he is fit to be the prime minister of this country.”

She said there was no need for her to resign as an MP to force a byelection because she had explicitly fought the 2017 election saying she did not support Corbyn, claiming she had won her seat in spite of him and “not on his coat-tails”.

Ryan also claimed there were “many very, very unhappy” MPs in Labour who could come to a similar decision and join the Independent Group of MPs.

The news of Ryan’s resignation emerged as Westminster was abuzz with rumours that a string of centrist Tories could be poised to join the Independent Group, which was formed on Monday by seven former Labour MPs, including Chuka Umunna and Chris Leslie.

The ardent anti-Brexit campaigners Anna Soubry and Sarah Wollaston, as well as Heidi Allen, who has been on an anti-poverty tour around Britain with the ex-Labour MP Frank Field, were deemed the most likely recruits, but did not respond to requests for comment.

Another vehement critic of Theresa May’s Brexit policy, Sam Gyimah, the former Tory minister who resigned over the government’s position on Brexit, ruled out joining the Independent Group.

“I hugely admire Luciana Berger’s brave stance against antisemitism. But, ultimately, this is a Labour party matter and is nothing to do with me,” he said.

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Corbyn had earlier sought to play down the significance of the seven Labour resignations, telling the group that they they won their seats in 2017 on Labour’s leftwing manifesto. “They were elected to carry out those policies. They decided to go somewhere else,” he told a manufacturing conference in Westminster.

Play Video 2:14 'We have all now resigned': seven Labour MPs quit party – video

Labour is keen to ramp up the pressure on the breakaway MPs to trigger byelections in their seats, and run against official Labour candidates.

In a signal of the leadership’s hardening attitude to the breakaway MPs, Corbyn’s close ally Jon Trickett, the shadow minister for the cabinet office, announced that Labour would consult on changing the law to allow constituents to remove an MP who resigns from their party.

“Communities should not have to wait for up to five years to act if they feel their MP is not properly representing their interests,” Trickett said. “This proposed reform has the dramatic potential to empower citizens and will be one of many measures the Labour party is planning to consult on and announce that will change the way politics in this country is done.”

Current law allows voters to petition for the recall of their MP, but only in specific circumstances – such as where they’ve committed a criminal offence, as in the case of the Peterborough MP Fiona Onasanya.

The former prime minister Sir John Major underscored the mood of political turmoil last night, when he became the latest senior Tory to warn that his party had been captured by pro-Brexit rightwingers.

“At the moment, there are people who – for now – may have their boots within the Conservative or Labour parties, but not their minds, nor their hearts,” the former Tory prime minister told an audience at the University of Glasgow on Tuesday, warning that both parties were being, “manipulated by fringe opinion”.

He suggested traditional mainstream Conservatives were being “hollowed out” by rightwing ex-Ukip members seeking to influence the government’s Brexit policy.

Corbyn’s unrepentant stance appeared unlikely to reassure a fresh wave of Labour MPs wondering whether to quit their party. Ian Austin, MP for Dudley South, told his local paper, the Express and Star: “People, me included, are going to be thinking long and hard about the position we’re in now.”

He said that extremism, antisemitism and intolerance had “driven good MPs and decent people who’ve committed their lives to mainstream politics out of the Labour party” and said he would make a decision about his own future by the end of the week.

Play Video 2:27 Tom Watson: Labour must tackle antisemitism to prevent more MPs leaving – video

When the Labour leader was asked on Tuesday whether he agreed with his deputy, Tom Watson, who said on Monday the frontbench of his party needed to “broaden out” to reflect the balance of views in the parliamentary Labour party, he said: “I recognise that leading the party means you have to take people with you, and I’m determined to do that.”

But he said Labour policy was already developed in consultation with backbenchers, adding: “Anyone who does not feel consulted is not taking up, in my view, the opportunities that are available, at all times, to do that.”

There was no hint that Corbyn was prepared to carry out the reshuffle Watson appeared to be demanding.

Corbyn’s tone was in marked contrast to that of John McDonnell, who promised a “mammoth, massive listening exercise”.

The shadow chancellor told BBC Radio 4’s Today programme: “If there are issues that we have to address, we’ll address them. If it is about the style of the leadership, we’ll address that. If it is about policy, we will listen to that as well.”

Major praised the courage of the breakaway Labour MPs; but said both main parties needed to remain “mainstream”.

He said Jacob Rees-Mogg’s pro-Brexit European Research Group (ERG) had become a party within a party, “with its own whips, its own funding and its own priorities”. He said: “Some of its more extreme members have little or no affinity to moderate, pragmatic and tolerant Conservatism.”

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Major has made a series of forthright interventions in the Brexit debate in recent months – but his remarks underlined the sense that with little more than six weeks to go until Britain is due to leave the EU, the ongoing wrangle about the shape of the Brexit deal is shaking up both political parties.

Tobias Ellwood, the defence minister, echoed Major’s concerns about the influence of the ERG on the government’s direction, telling Sky News: “There are many of us who normally would not be commenting in public about a wing of our party or, indeed, individuals themselves.

“We are doing so because they are coming to the point of tarnishing the actual brand of the party and I want to remain inside a modern, compassionate, inclusive, outward-looking party that’s attractive to the next generation.”

He said a series of senior Conservatives had decided to come forward to say “no, not in our name,” adding that he didn’t want the approach of the ERG to “poison the moderate perspective of the centre-right Conservative party”.