Jewish organisations have called on Labour to distance itself from its Peterborough byelection candidate, who has apologised for liking a post that said Theresa May had a “Zionist slave masters agenda”.

Lisa Forbes, who will fight the marginal seat for Labour in the knife-edge byelection on Thursday, said she apologised “wholeheartedly for not calling out these posts”.

She claimed she had “intended to like a video of schoolchildren praying in solidarity with the Christchurch attacks, not the views expressed in the accompanying text”.

The Twitter activity was unearthed by a freelance reporter, who also found Forbes had commented on a thread that claimed Isis was created and funded by the CIA and Mossad. Forbes wrote: “I have enjoyed reading this thread so much.”

In a joint statement, Marie van der Zyl, the president of the Board of Deputies of British Jews, Simon Johnson, the chief executive of the Jewish Leadership Council, and David Delew, chief executive of the Community Security Trust, said it “brings into question her suitability for public office”.

The trio said it “stretches the limits of credulity” for Forbes to claim she had not been aware of the comments when she liked the post.

“We are fed up hearing that Labour opposes antisemitism while repeatedly hearing excuses that its members accidentally missed the racism that was staring them in the face,” the statement said. “Unless Labour disowns Lisa Forbes as a candidate, it will only confirm the party’s shameful descent into the racist mess for which they are now being investigated by the Equality and Human Rights Commission.”

Forbes said she would undertake antisemitism training regardless of whether she was elected. “Antisemitism is abhorrent and has no place in our society,” she said. “Standing up against hatred towards one group of people must never be allowed to become hatred of another.

“Regardless of whether I am elected, I will deepen my understanding of antisemitism, so I can act as an ally, challenging antisemitism wherever it occurs.”

Labour has poured huge campaigning resources into the seat, and Jeremy Corbyn led a rally in Peterborough in support of Forbes on Saturday. The grassroots pro-Corbyn group Momentum has said it hopes to get at least 2,000 activists out in the constituency in the run-up to the election on Thursday, as well as mobilising at least 4,000 local members via its phone bank.

The criticism comes after a week of difficult revelations for Labour. The EHRC announced it would launch an investigation into antisemitism in the party, and Labour was forced to suspend a member of its governing body, Pete Willsman.

Willsman, a member of the national executive committee, suggested in recorded comments that complaints of antisemitism were being whipped up by Israel, and that 68 rabbis who wrote to the Guardian about their concerns had been co-ordinated by the Israeli embassy.

The Peterborough byelection is taking place because the former Labour MP Fiona Onasanya was subject to a recall petition after being found guilty of lying about a speeding ticket. The election is expected to be a three-way fight between Labour, the Conservatives and the Brexit party.

The Brexit party candidate, Mike Greene, is favourite to win the seat, which would be the first for Nigel Farage’s party. It was won by Labour from the Conservatives’ Stewart Jackson by 607 votes in 2017, but Onasanya was expelled from Labour after her conviction. She was the first MP to be ousted using a recall petition.

More than one in four registered voters – 27.64% – signed an official notice calling for her removal, according to Peterborough city council. This was well above the 10% threshold.

Conservative leadership candidates have also descended on Peterborough to campaign for their local candidate, Paul Bristow, including Boris Johnson, Jeremy Hunt and Michael Gove.