However, he will still appear on the ballot as the party's candidate on Thursday 29 February.

Politics latest: Labour in 'absolute mess' over by-election

So, what happened and why can't he be replaced?

Mr Ali's candidacy for the by-election was thrown into the spotlight after reports he told a meeting of the Lancashire Labour Party that Israel deliberately allowed the Hamas 7 October attacks to take place in order to give it the "green light" to invade Gaza.

He issued an "unreserved" apology for the "deeply offensive, ignorant and false" remarks and Labour initially stood by him, saying he had "fallen for a conspiracy theory".

But less than 48 hours later, further comments came to light and Labour withdrew support for the candidate.

A report in the Daily Mail on Monday evening claimed Mr Ali blamed "people in the media from certain Jewish quarters" for fuelling criticism of a pro-Palestinian Labour MP, as well as claiming Israel planned to "get rid of [Palestinians] from Gaza" and "grab" some of the land.

A party spokesperson said: "Following new information about further comments made by Azhar Ali coming to light today, the Labour Party has withdrawn its support for Azhar Ali as our candidate in the Rochdale by-election."

Labour acknowledged the circumstances were "highly unusual".

The spokesperson added: "Given that nominations have now closed, Azhar Ali cannot be replaced as the candidate."

Why can't Labour replace Ali?

According to Electoral Commission rules, a candidate can only pull out of a race if they submit a withdrawal notice by the deadline for withdrawals (which is by 4pm, 19 working days before the poll).

The deadline for the Rochdale by-election passed on 2 February.

That means Mr Ali will appear on the ballot paper as the Labour candidate when voters go to the polls.

What happens if Ali wins?

However, if Mr Ali wins the by-election, he will not become a Labour MP. Instead, he will sit as an independent.

That's because by withdrawing support, Labour has effectively removed the whip were he to be selected as the constituency's next MP.

Having the whip withdrawn is a severe punishment which means you are kicked out of the parliamentary party.

Other former Labour MPs this has happened to include former party leader Jeremy Corbyn and veteran left-wing MP Diane Abbott.

Both of those cases were to do with rows over antisemitism, which Sir Keir Starmer has promised to tear out of his party following the controversies that dogged the Corbyn era.

Why is there a by-election and who else is standing?

The decision to withdraw backing for Mr Ali will come as a blow to Labour, which had hoped to retain the seat following the death last month of veteran sitting MP Sir Tony Lloyd.

It also means Labour will need to find a new candidate to contest the seat at the upcoming general election.

Also running in Rochdale are former Labour MP Simon Danczuk, now the Reform Party candidate, and George Galloway, of the Workers Party of Britain, who is campaigning against Labour's stance on Gaza.

What questions remain for Sir Keir Starmer?

The party leadership is being pressed on why Mr Ali was not immediately suspended after the initial comments emerged.

Labour recently suspended MP Kate Osamor after she appeared to say the Gaza war should be remembered as genocide on Holocaust Memorial Day.

And Sir Keir has repeatedly promised to tear antisemitism out "by its roots" in Labour.

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Starmer's biggest crisis - and there may be worse to come

A spokesperson for the Campaign Against Antisemitism said that rather than appearing as a principled decision, Labour's withdrawal of support "looks as expedient as the failed attempt to defend him".

Meanwhile, Martin Forde KC, who led a review into the party's culture under Mr Corbyn, said Labour MPs feel there has been a "disparity in treatment" of allegations of antisemitism within the party.

He told BBC Radio 4's Today programme: "One does have to question how such individuals are selected in the first place, and also the disparity in treatment, because one of the things that concerned us when we talked about weaponisation was certainly the perception that antisemitism was along factional lines."

How are candidates selected?

Mr Ali, the leader of Lancashire County Council's Labour group, was chosen by local party members last month after making it on to the candidate shortlist with two other people - political journalist Paul Waugh and Wigan councillor Nazia Rehman.

Lord Mann, the government's antisemitism adviser and a former Labour MP, told Sky News the filters in place during Labour's selection process were "clearly not good enough" and Sir Keir will likely be "fuming".

He added that "heads may roll" over the decision to shortlist Mr Ali, and the move to withdraw support was "bold and brave".

He said this has "never happened before" in a by-election, adding: "I think the Jewish community, as it reflects on this, will take great comfort in the fact that Keir Starmer has been prepared to do it. It's certainly a bold move."