A leading Jewish academic has threatened the University and College Union with legal action as the backlash against its rejection of the widely-accepted definition of antisemitism reached new levels this week.

Lawyers acting for Ronnie Fraser, director of Academic Friends of Israel, have written to UCU general secretary Sally Hunt alleging the Union's stance breaches equality laws.

Mr Fraser, a freelance maths lecturer who has been a member of the union for 11 years, demanded that UCU should "acknowledge it has been guilty of institutional antisemitism" and publicly apologise to its Jewish members and ex-members.

Four more Jewish academics resigned from the UCU on Tuesday. The quartet - all representatives of the Scottish Council for Jewish Communities - said UCU's "racist policy towards Jews" was incompatible with their membership.

Some Jewish academics have said that if Mr Fraser's case goes to trial it could be as significant as the Irving-Lipstadt case of 2000, when convicted Holocaust denier David Irving lost a libel suit.

Anthony Julius, of law firm Mishcon de Reya, is representing Mr Fraser. He also defended Professor Lipstadt and said he could understand why people were comparing the cases.

"In terms of the witnesses and issues raised it seems a fair analogy," he said.

"We sent UCU a serious letter. There's no element of bluff involved. The threat does not exceed the reality. The letter was sent in the full expectation that there's something that could go to trial and we are ready for that."

In the letter, Mr Julius, who is chairman of the JC, accuses UCU of "harassing" Mr Fraser and violating his dignity by creating "an intimidating, hostile, degrading, humiliating and offensive environment...in simple terms, the UCU is not a place that is hospitable to Jews. This is not just a violation of equality legislation, it is also a scandal".

The letter claims UCU intensified its conduct each year by "piling fresh insults" on its Jewish members.

Mr Fraser has also demanded that the union repeals its resolution rejecting the definition of antisemitism, commits to a code of conduct, sets up an inquiry into its "institutional antisemitism" and sponsors a programme to educate academics about the dangers of antisemitism. UCU has not yet responded to Mr Fraser, and has repeatedly refused to comment on the matter. Mr Fraser is expected to start tribunal proceedings if the union fails to reply by August 5, or fails to meet his demands.

His spokesman said: "As a member of the union for 11 years, Ronnie Fraser has decided enough is enough. Having experienced various instances of institutionally antisemitic behaviour by the UCU over the years, the passing of the resolution in Harrogate was the final straw. Countless attempts at education and constructive debate by the AFI have fallen on deaf ears, forcing him to take this action."

The four ScoJec resignations came in a separate letter to Ms Hunt on Tuesday.

Ephraim Borowski, a former president of the Glasgow Association of University Teachers, (the AUT is one of the two components which make up the UCU) and his colleagues, wrote: "We have grave concerns. The racist propaganda brought in the wake of the Middle East crisis has exposed Jewish people in Scotland and the UK to a wave of hostility. This is the situation you are feeding."

The academics said UCU's stance could impinge on Scottish Jews and cited the possibility of Jewish students facing anti-Israel attacks but not receiving support from UCU-supporting lecturers.

David Hirsh, of the anti-boycott Engage website, said UCU now found itself "between a rock and a hard place".

In a blog examining Mr Fraser's threatened action, Mr Hirsh wrote: "There are two factions inside the decision-making structures of the union. There are the hardcore anti-Zionists and then there are the grown-ups.

"The anti-Zionists will storm with anger that UCU is being sued. They will say that the Israel lobby is conspiring against the union, that it is hugely powerful, that it is in cahoots with those who want to privatise education, that it is playing the antisemitism card in bad faith and that it is putting trade union solidarity at risk.

"The grown-ups, including the trustees and the lawyers who will advise the leadership, will want to settle this court action and to make it go away."

Communities Secretary Eric Pickles last week attacked the Union, saying its Jewish members had become "fair game for invective".