At least three Conservative MPs were on defection-watch on Wednesday morning, hours after the Labour MP Joan Ryan became the eighth in her party to resign and join the breakaway Independent Group.

The ardent anti-Brexit campaigners Anna Soubry and Sarah Wollaston, as well as Heidi Allen, who has been an outspoken Tory voice against austerity, have all been tipped as MPs who could join the group of Labour defectors, who include Luciana Berger and Chuka Umunna.

On Tuesday night, another vehement critic of Theresa May’s Brexit policy, Sam Gyimah, who resigned as a minister 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. Others who have previously been critical of the Conservatives’ approach to Brexit, including Nick Boles and Antoinette Sandbach, both ruled out quitting the party.

Phillip Lee, who quit as justice minister to back a second referendum, had also been tipped as a potential Tory departure but ruled out the move on Wednesday morning.

Lee, the chair of Right to Vote campaign for a new referendum, said he had warned the prime minister of possible defections if she did not respond to their letter asking to meet. “I’m certainly not leaving the Conservative party but I could understand if others did,” he told Sky News.

The former Tory chancellor Ken Clarke said on Wednesday he believed some Tory MPs were considering joining the Independent Group, which is expected to be a precursor to a new political party.

“Certainly some members of parliament are getting very fed up,” he told BBC Radio 4’s Today programme.

“There are some, not including me, who probably are contemplating leaving if the party moves too far to the right and no longer represents what they regard as the mainstream Conservative views they have held for all the previous years. I hope that doesn’t happen. I hope it doesn’t come to that.”

The former prime minister Sir John Major also warned that his party had been captured by pro-Brexit rightwingers and praised the courage of the breakaway Labour MPs

“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,” he told an audience at the University of Glasgow on Tuesday, warning that both parties were being, “manipulated by fringe opinion”.

Ryan, the MP for Enfield North, said she had been a Labour member for four decades but blamed her departure on what she claimed was the party leadership’s “dereliction of duty” in tackling antisemitism.

Mike Gapes, Gavin Shuker, Angela Smith, Ann Coffey and Chris Leslie make up the rest of the Independent Group.

Ryan claimed there were “many very, very unhappy” MPs in Labour who could come to a similar decision and join their eight former colleagues. Ian Austin, MP for Dudley South, told his local paper, the Express and Star, he would make a decision about his own future by the end of the week.

“People, me included, are going to be thinking long and hard about the position we’re in now,” he said.

On Tuesday night, 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,” he 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.”