The sacking of Azhar Ali as Labour’s candidate in the Rochdale by-election is the biggest disaster for Sir Keir Starmer in his nearly four years as leader of the party.

There will now be an almighty inquest into how a candidate who had made such comments about Israel and Gaza was selected for such a high-profile by-election.

Defending Rochdale was never going to be easy for Labour. Sir Tony Lloyd’s majority at the 2019 general election was 9,668, with the Conservatives in second place.

But the challenge from firebrand left-winger George Galloway and the constituency’s former Labour MP Simon Danczuk standing for Reform UK already presented difficulties.

But now Labour goes into the by-election on 29 February with no candidate, even though Mr Ali’s name will still be on the ballot paper, right at the top of the list in alphabetical order.

Follow latest: Labour withdraws support for Rochdale candidate

Why was he selected? Not surprisingly, as happens in many constituencies with a large ethnic minority population, the local Asian community will have wanted one of their own to represent them.

Mr Ali faced a strong challenge, however, from political journalist Paul Waugh, who was seen as the preferred choice of Sir Keir and the Labour leadership. It was something of a surprise when he wasn’t selected.

It has been reported that voting at the selection meeting was 87 votes for Mr Ali to 68 for Mr Waugh. Hardly a resounding victory for Mr Ali.

And Mr Waugh, as an experienced Westminster operator, would surely have been a safe pair of hands.

But the Labour activists who chose Mr Ali weren’t were not selecting a political novice. He’s the leader of the Labour group on Lancashire County Council and he’s been a parliamentary candidate twice, in his home town of Pendle.

He stood against Tory MP Andrew Stephenson in 2015 and 2019, coming second, 6,186 votes behind Mr Stephenson in 2019. He also acted as an adviser to the Blair and Brown governments between 2005 and 2010.

He has nearly 25 years’ experience in local government, but alarm bells should have rung in the Labour high command over his backing for Jeremy Corbyn in the 2016 Labour leadership election.

That’s despite saying Mr Corbyn would be a disaster for the country in 2015, when Mr Corbyn first stood for leadership after the resignation of Ed Miliband.

So if the first Labour blunder was selecting Mr Ali in the first place, the second was standing by him for 48 hours after his comments about the 7 October attacks in Israel were first reported in the Mail on Sunday.

A third blunder was failing to discover that it wasn’t just the Mail on Sunday disclosure that was embarrassing. Labour now admits “new information” and “further comments” have come to light.

Senior Tories claim the disclosure of his controversial remarks prove that Sir Keir’s claims – made repeatedly in Prime Minister’s Questions and elsewhere in recent weeks – were a hollow sham.

On the Labour left, meanwhile, they claim it was grossly unfair that Sir Keir and the leadership backed Mr Ali when left-wingers Kate Osamor, Andy McDonald and Mr Corbyn himself had been suspended over antisemitism allegations.

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But there may be worse to come for Labour. Suddenly, the Rochdale disaster – 17 days before polling day in that by-election – plunges the two by-elections this week, in Wellingborough and Kingswood, into potential disarray.

Until this weekend, bookies and pollsters had been predicting victory for Labour in both seats, even though Labour is fighting to overturn big Conservative majorities.

Now, any senior Labour figures campaigning in the final days ahead of polling day this Thursday – and indeed the candidates themselves – will be besieged by questions about Mr Ali and the Rochdale fiasco.

Andy Burnham, mayor of Greater Manchester, deputy party leader Angela Rayner and leading shadow cabinet member Lisa Nandy have all made high-profile visits to Rochdale to support Mr Ali in the past week.

The Tories, of course, will be cock-a-hoop. A week in which an embattled Rishi Sunak was facing a rise in inflation, two potential by-election wipe-outs and renewed plotting against him by Tory MPs has now turned into Christmas, Easter and his birthday all at once.

The veteran Labour MP John McDonnell told Sky News he couldn’t recall the sacking of a by-election candidate during a campaign before. And, in covering by-elections for 40 years, nor can I.

This is far worse for Sir Keir and Labour than the “Red wall” Hartlepool by-election defeat in 2021 and the ULEZ backlash that handed victory to the Tories in Boris Johnson’s Uxbridge and South Ruislip constituency in July.

Labour’s lead in the opinion polls has remained stubbornly around the 20% mark for months. That could now change.

Will we see the beginning of the end of Labour’s seemingly unassailable poll lead?

Is this a turning point for the Tories? Possibly. They won’t win Rochdale, but they won’t care about that. Humiliation for Sir Keir Starmer is a massive gift for the Conservatives.

And despite all the talk from the likes of campaign chief Pat McFadden that Sir Keir has taken a tough decision, it doesn’t look like that.

It’s more than an embarrassing retreat and U-turn by the Labour leader. It’s also an unmitigated disaster for Labour and Sir Keir’s worst crisis in his time as leader.