Mr Serwotka, who leads the Public and Commercial Services union (PCS), told a fringe event at the Trades Union Congress (TUC) conference on Tuesday that the Jewish state could have “created a story that does not exist” in order to distract attention from “atrocities” he said it has committed.
The comments, revealed by The Independent, come after Labour was engulfed in a bitter row in the summer on its handling of allegations of antisemitism in its ranks.
Labour has refused to comment on the remarks, or to respond to questions on whether the party would launch an investigation into Mr Serwotka’s conduct.
The TUC also did not directly condemn Mr Serwotka’s comments and failed to respond to questions over whether he would remain president of the umbrella body, a position he was elected to this week.
Asked about his remarks, a spokesperson said: “The TUC opposes antisemitism and discrimination of all kinds, as do our member unions. All TUC unions have a commitment to tackling racism and promoting equality in their rules.
“If anyone experiences antisemitism, or any form of discrimination in the union movement, they should be confident that they can raise it with their union, and their union will take their concerns seriously.”
The row centres on remarks made by Mr Serwotka at a Palestine Solidarity Campaign event, where said he “deplored” antisemitism but claimed accusations against Jeremy Corbyn were the result of “something sinister going on”.
He said: “I think it is unfortunate that the Labour Party allowed a lot of this to drag on in a way that actually did not help anybody.
“In a year when Donald Trump has moved the US embassy from Tel Aviv to Jerusalem, in a year when dozens of Palestinians including children were gunned down – unarmed innocent civilians – by the Israeli military, in a year when the Americans are cutting off aid … isn’t it a vile world when, instead of being on the front foot, denouncing these atrocities, demanding an independent and sovereign state for the Palestinian people, we have had a summer of asking ourselves whether leading Labour movement people are in any way antisemitic?”
He added: “I’m not a conspiracy theorist, but I’ll tell you what – one of the best forms of trying to hide from the atrocities that you are committing is to go on the offensive and actually create a story that does not exist for people on this platform, the trade union movement or, I have to say, for the leader of the Labour Party.”
Euan Philipps, a spokesperson for Labour Against Antisemitism, said the speech was “a stark illustration of how deeply embedded antisemitism is within the Labour movement” and called for Mr Serwotka to resign as PCS general secretary.
However, the union has stood by him, with a spokesperson saying: “Mark spoke at a Palestine Solidarity Campaign fringe event at the TUC – an organisation PCS is affiliated to.
“He made the point at the start of the meeting that we need to oppose antisemitism in society and within the Labour movement.
“But we should not allow the issue of antisemitism to be used by people who are attempting to silence Palestinian voices as they legitimately struggle for their rights and a sovereign state.”
The Independent has launched its #FinalSay campaign to demand that voters are given a voice on the final Brexit deal.