For Labour Party managers, the disclosure that Rochdale by-election candidate Azhar Ali had been recorded saying that Israel let Hamas perpetrate the October 7 massacre to create a pretext for invading Gaza was a nightmare come true – for two reasons.

The first was that it forced the party to suspend him, paving the way for the pro-Palestinian insurgent George Galloway to seize the seat.

But according to party sources, the second consequence was equally horrendous: it indicated that Ali had slipped through their “due diligence” net – a system thought to be the most rigorous in Labour history – which in turn delayed their election preparation.

“Ali’s case delayed the continuing process of approving general election candidates by about a month,” a senior Labour official, speaking on condition of anonymity, told the JC. “We realised there had to be even stricter due diligence than we already had.”

But the process, he went on, was now robust. In 1997, when Tony Blair won a landslide majority with 145 more Labour MPs than there had been before the election, the party knew little about some of their backgrounds. This, he maintained, was an error it would not repeat.

Lord Peter Mandelson, a New Labour veteran who remains highly influential, agreed, saying the discipline now being exercised was unprecedented, especially over issues that concern Jews.

“Given the whole Corbyn experience, you can’t blame Starmer for adopting a more thorough approach to candidate selection. This hasn’t replaced the democratic process in each constituency, but it has added a layer of caution about those who come from the hard left or choose to tolerate or express antisemitic views. After Rochdale, this has only become more intense,” he said.

Supervising the vetting are three-person panels drawn from Labour’s National Executive, which draw up longlists for every “battleground” and winnable seat.

Following this, the party official said, staff based in regional offices and at its national headquarters comb potential candidates’ social media feeds, using search terms to find inflammatory posts – for example, the word “Zionist”, because “people often use this term when they want to be disparaging”. Along with original posts, “likes” and shares will also be picked up.

The official said that candidates’ political records will also be carefully scrutinised: “If they’ve served as councillors, we look to see if they’ve ever rebelled against their whip. Have they ever slagged off Keir Starmer, or made a public political pledge that might now prove embarrassing?” They will also be required to disclose their tax records.

As for antisemitism, candidates found to have made statements that fit the International Holocaust Remembrance Association definition, such as suggesting that Israel has no right to exist or is comparable to Nazi Germany, “would not be allowed to run, and instead, would be investigated”.

A candidate who had expressed support for the BDS campaign or joined a chant of “river to the sea” might not automatically be disqualified, depending on what they said when questioned about it. 

According to the official, the consequence of all this was that in the event of a 1997-style landslide, the left would be far weaker inside Parliament.

He named five candidates, all selected after the 2019 election before Starmer’s allies had fully taken control, that he thought would be likely to join the hard-left Campaign Group. But with the departure of members such as Corbyn, he said it would be no bigger than its current strength of 35 – and swamped by a parliamentary party whose total of 199 MPs could double.