Labour has told its activists they face "disciplinary action" if they continue to campaign for the party's abandoned Rochdale by-election candidate Azhar Ali.
The party's branch in the North West has emailed activists in the region to warn they must no longer canvass for Mr Ali now that Labour has withdrawn its support for his candidacy over alleged antisemitic remarks he made last year.
The party's rulebook states that support for any political organisation or candidate that is not Labour is prohibited.
Mr Ali first faced criticism when the Mail on Sunday reported that he had told a meeting of community activists last year that Israel deliberately allowed the Hamas atrocity to take place in order to give it the "green light" to invade Gaza.
A number of shadow ministers initially stood by Mr Ali and said he would remain the candidate in light of his "unreserved" apology for the "deeply offensive, ignorant and false" comments.
However, the party withdrew support for Mr Ali when further remarks were published by the Daily Mail in which he allegedly 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.
It also temporarily suspended him from the party pending investigation.
On Tuesday night, Labour suspended another of its prospective parliamentary candidates, Graham Jones, after he allegedly referred to "f***ing Israel" at the same meeting where Mr Ali is reported to have made his comments.
In the email to Labour members, seen by Sky News, the party writes: "As you will be aware, the Labour Party has withdrawn its support for Azhar Ali as the Labour Party's candidate and the Labour Party campaign in Rochdale has now ceased.
"In view of these developments, we must inform you that members are not permitted to campaign in the by-election on behalf of Azhar Ali.
"If members are found to do so, they will be subject to disciplinary action in accordance with the Labour Party's rules."
It added: "We understand that this news may come as a disappointment to many of you.
"We would like to thank you for your hard work in challenging circumstances."
Sir Keir Starmer insisted on Tuesday that he had taken "decisive action" to withdraw support for Mr Ali, saying: "It is virtually unprecedented to withdraw support for a candidate in the way that I withdrew support for this candidate yesterday.
"That's what a changed Labour Party is all about."
However, the Labour leader has been criticised for not acting immediately after Mr Ali's remarks came to light, with housing minister Lee Rowley telling Sky News the episode showed "the Labour Party is in a real mess".
"It is just extraordinary to see some of the things that are coming out now, some of the conspiracy theories," he told Breakfast With Kay Burley.
"It is showing that the Labour Party really hasn't got a grip on this, that it is a party which has not changed [and] a party which will say anything to win government."
Sir Keir's decision to withdraw support for Mr Ali creates an unusual situation whereby he will still appear on the ballot paper as the Labour candidate - because it is too late for his name to be removed - although the party is not backing him.
Labour has also confirmed that in the event Mr Ali wins the contest on 29 February, he will sit as an independent MP on entering the Commons.
The incident with Mr Ali has also angered those on the left of the party, with campaign group Momentum accusing Sir Keir of "double standards" and " trying to save one of their own".
Mr Ali led the Labour group on Lancashire County Council before being selected as a candidate for the Rochdale by-election, set for 29 February.
He was hoping to replace Labour stalwart Sir Tony Lloyd, who died in January, and had been the Labour MP since 2017.
See below the full list of candidates and the political parties they represent:
Azhar Ali, (listed as Labour Party, but now removed as its candidate)
Mark Coleman, Independent
Simon Danczuk, Reform UK
Iain Donaldson, Liberal Democrats
Paul Ellison, The Conservative Party Candidate
George Galloway, Workers' Party of Britain
Michael Howarth, Independent
William Howarth, Independent
Guy Otten, Green Party
Ravin Subortna, The Official Monster Raving Loony Party
David Tully, Independent