Unite general secretary Len McCluskey said such actions were "despicable" but added that the party "never handled the anti-Semitism issue correctly".

Labour's response to alleged anti-Semitism among its members has been under investigation by the Equality and Human Rights Commission since last May.

Meanwhile, his union has backed Rebecca Long-Bailey for Labour leadership.

But Mr McCluskey said it was "unfair" to describe her as the "continuity Corbyn" candidate as she will have different priorities.

Shadow Brexit secretary Sir Keir Starmer, Wigan MP Lisa Nandy and shadow foreign secretary Emily Thornberry are also running for the Labour leadership.

Speaking to the BBC's Andrew Marr Show, Mr McCluskey said: "I'm absolutely convinced that there were those individuals who opposed Jeremy Corbyn's election right from the beginning, used the anti-Semitism issue - which I think is quite despicable that they did this on such an important subject - to undermine Corbyn, there's no doubt about that."

But he said that the party had "never handled the anti-Semitism issue correctly", adding: "We should have done things quicker."

The Equality and Human Rights Commission is assessing whether Labour has "unlawfully discriminated against, harassed or victimised people because they are Jewish".

Labour has been plagued by accusations of anti-Semitism since mid-2016 - and the leadership has been accused by some of its own MPs of tolerating a culture of anti-Jewish prejudice.

Leader Jeremy Corbyn has insisted he is getting to grips with the issue and the party's internal disciplinary procedures have been beefed up.

On Labour's losses in December's general election, Mr McCluskey said the party had "suffered the consequences" of "being perceived as a Remain party", adding that the timing of the poll was "a trap".

He said that Theresa May's Brexit deal - which was rejected by MPs - "probably provided enough issues to try and reach a compromise".

Earlier, he told Sky News' Sophy Ridge On Sunday that Labour's election defeat was "virtually, solely down to Brexit" and defended the party's policies.

But others criticised Mr Corbyn's leadership in the wake of Labour's fourth general election loss in a row.

Immediately after the election result, former Labour MP John Mann said the leader's unpopularity on the doorstep was palpable and Labour's Caroline Flint, who lost her seat in Don Valley, said many of her voters "could not and did not want to support Jeremy Corbyn".