Anti-Semitism is a problem in the Labour Party and the leadership must tackle the issue “head on” by adopting the international definition in full, a senior union boss has said.

GMB general secretary Tim Roache said it is “abundantly clear” that Labour’s code of conduct on anti-Semitism must be rewritten to include all elements of the International Holocaust Remembrance Alliance definition to bring an end to current infighting.

Jeremy Corbyn has so far resisted massive pressure to incorporate the IHRA list of examples of anti-Semitic behaviour unamended in the code, approved by Labour’s ruling National Executive Committee last month.

And calls by deputy leader Tom Watson for the list to be adopted in full led to a furious response from some activists, with the hashtag #resignwatson trending on Twitter.

Labour argues that anti-Semitic behaviours omitted from its list are covered elsewhere in the document in a way which will make it easier to take disciplinary action.

Critics of the IHRA definition warn that some of its examples could inhibit criticism of the actions of the Israeli government.

But MPs are expected to vote in September for the definition to be adopted in full. And the the Jewish News reported that Momentum founder Jon Lansman had been lobbying the party’s leadership for weeks to take this step.

In an article for the Huffington Post, Mr Roache said he is in “despair” about members of the party “turning on each other”.

“Instead of taking the fight to a divided and squabbling Tory Party, we’re knocking lumps out of each other,” said the GMB boss. “It has to end.”

Mr Roache said: “Anti-Semitism is a problem in the Labour Party. I’ve been shocked at what I’ve seen on social media.

“For my entire life, and with every fibre of my being, I have campaigned against and fought racism. There is no place for anti-Semitism in our party, we need to tackle that head on as well as providing political education and training for members.

“The Labour movement owes a lot to the contribution of Jewish members and campaigners and the party needs to get to work to rebuild trust across our Jewish communities.

“It is abundantly clear that Labour has to accept IHRA examples of anti-Semitism in full, while agreeing that criticising the Israeli government and supporting our Palestinian brothers and sisters is not being anti-Semitic.

“As one MP told me `criticise the Israeli government, but let’s do it without being a racist and without being an arse’. Surely, as a movement, that low bar cannot be beyond our ability?”

Mr Roache dismissed talk of a breakaway party for centrists dismayed by Mr Corbyn’s handling of the issue.

“An SDP split helps no-one except the right, as it did with Thatcher in the 1980s,” he said.

But he warned that Labour risks letting down its voters if it chooses infighting over unity.

“We can make the choice to be the Labour Party, an inclusive broad church and determined Government in waiting, or we can choose to let down the people who need us most through our own indulgence or settling of old scores,” he said.

Mr Roache’s intervention came as shadow chancellor John McDonnell faced criticism over a 2012 speech in which he described Israeli attacks on Gaza as attempted genocide.

Mr McDonnell’s comments were highlighted by the Daily Telegraph, which also drew attention to his 2016 appearance on a platform alongside activist Jackie Walker, who was suspended by Labour for alleged anti-Semitism.

Speaking to a Unite The Resistance event at a time of intensive Israeli air strikes on the Gaza enclave, Mr McDonnell said: “It’s absolutely critical now that we use every platform we can to expose what’s going on, which is effectively an attempt at genocide against the Palestinians.”

Karen Pollock, chief executive of the Holocaust Educational Trust, told the paper his remarks were “not only inaccurate, but irresponsible and deeply offensive”.

But Mr McDonnell accused the Telegraph of “doing a number” on him and made clear he stands by the remarks.

He released a statement saying: “Our response: ‘John takes pride in and stands by his track record of forcefully and justifiably condemning brutal attacks on the Palestinian men, women and children of Gaza  and will always stand up for victims of such disproportionate violence’.”