Len McCluskey, the leader of Britain’s largest trade union, has accused Labour MPs of “smearing” Jeremy Corbyn over anti-Semitism. In an article in tomorrow’s New Statesman, the Unite general secretary writes that “this issue has joined a line of others in being used by a group of backbench Labour MPs to attack and undermine Jeremy Corbyn”.

McCluskey, who has been a close ally of Corbyn since the Labour leader was elected in 2015, adds: “I look with disgust at the behaviour of the Corbyn-hater MPs who join forces with the most reactionary elements of the media establishment and I understand why there is a growing demand for mandatory reselection.”

The Unite leader’s article comes after Corbyn acknowledged “the uncomfortable fact” that some Labour members and supporters hold anti-Semitic views. The founder and chair of the pro-Corbyn grassroots organisation Momentum, Jon Lansman, said late last year that Labour must do more to tackle anti-Semitism in the party. In a recent parliamentary debate, Jewish Labour MPs shared examples of the abuse they had received. Earlier today, 50 Labour MPs and peers accompanied the Jewish Labour MP Ruth Smeeth when she went to give evidence against an activist she has accused of anti-Semitism.

In a dramatic intervention in Labour’s ongoing feuds, McCluskey:

• Accuses the Israeli Labour Party leader, Avi Gabbay, of a “cynical and outrageous smear” against Corbyn. Gabbay recently severed relations with Corbyn’s office in response to “the hostility that you have shown to the Jewish community and the anti-Semitic statements and actions you have allowed”. This, McCluskey writes, was a “disgusting libel of which Gabbay should be ashamed”.

• Condemns the “few” anti-Semites in Labour (“any is too many”) and argues that “combating their views is not merely legitimate, but essential”. McCluskey adds: “I have fought anti-Semitism and anti-Semites all my life, including physically on the streets on occasion, and I need no lectures from anyone else on the subject. I am not sure that some of the voluble backbench critics of Jeremy Corbyn can say as much.”

• Denounces Labour MPs “such as Chris Leslie, Neil Coyle (my own MP), John Woodcock, Wes Streeting, Ian Austin and others” as “a dismal chorus whose every dirge makes winning a Labour government more difficult”. He accuses them of “working overtime trying to present the Labour Party as a morass of misogyny, anti-Semitism and bullying”.

• Writes that he “understands” why there is “a growing demand for mandatory reselection” of Labour MPs and warns that “promiscuous critics” who “wish to hold Corbyn to account can expect to be held to account themselves”.

Unite, which has donated £11m to Labour since Corbyn’s election and has 1.4 million members, is the party’s largest financial backer and its most powerful affiliate. Jennie Formby, the union’s former political director, recently became Labour’s general secretary and Andrew Murray, McCluskey’s chief of staff, serves as a part-time consultant to Corbyn. Karie Murphy, the Labour leader’s office director, is also a close ally of McCluskey.

The Unite general secretary, 67, has served as the union’s head since 2011. He was most recently re-elected in 2017 against his rival Gerard Coyne, by 45.4 per cent to 41.5 per cent. Unite endorsed Corbyn during the 2015 leadership election and was crucial to his survival during the 2016 coup attempt.

Though McCluskey writes that Labour MPs “have a right to express their own views”, he adds: “You would have to go back a long way to find such a sustained smearing by MPs of their own leader and their own party as we are seeing now.”

McCluskey continues: “Their determination to divide the party into pro- and anti-Corbyn factions, despite the huge increase in Labour’s vote secured last year... ultimately pollutes everything it touches. That includes the work against anti-Semitism”.

He laments that he “was wrong” to assume that “after the great advances in last year’s general election under Jeremy’s leadership”, Labour rebels would demonstrate loyalty to Corbyn. “To watch as these so-called social democrats tried to demean and attack, in front of our enemy, a decent and honourable man who has fought racism and anti-Semitism all his life and who has breathed life and hope back into the hearts of millions, especially the young, made my stomach churn. To see Tory MPs cheer and applaud them was shameful.”

McCluskey is currently facing a legal challenge by Gerard Coyne, his former leadership rival, who accuses him of breaching election rules by calling a contest when there was no vacancy. Should Jeffrey Burke QC, a retired high court judge, rule in Coyne's favour, the 2017 general secretary result will be annulled and a re-run held.