Labour Peer Lord Alan Sugar has urged Londoners not to vote for the party's candidate, Ken Livingstone, in the London Mayor elections.
Tweeting to his nearly 1.8 million followers, Sugar wrote: "I don't care if Ed Miliband is backing Livingstone. I seriously suggest no one votes for Livingstone in the mayoral elections."
"Sky doing live mayoral interview with candidates at 8pm today. Johnston v Livingstone is real issue. Livingstone must not get in on 3 May."
The Apprentice star is one of the party's more high-profile donors and supporters. He gave a total of £69,424 to Labour or to Labour leader Ed Miliband's office in 2011, including £12,576 as recently as December.
Sugar is a prominent member of the Jewish community and was believed to have been angry at Livingstone's recent comments, which suggested Jewish people would not vote for labour because they are rich.
Further clarifying his position, Sugar later tweeted: "I am not voting in Mayoral elections as (a) I am registered in Essex and ( b) I don't think Lords are allowed to vote."
Fellow Twitter users responded to the business mogul with both support and condemnation.
Paul Waugh, Editor of PoliticsHome.com, wrote: "Will Labour HQ dare to reprimand @Lord_Sugar for urging voters to back a rival to an official Labour candidate? #justaskin".
Owen Jones, Independent columnist and author of Chavs: The Demonization of the Working Class, tweeted: "It's up to the millions who is [sic] elected London's Mayor, not millionaire ex-Thatcher protegees. Thanks for the advice though @Lord_Sugar".
This is the latest setback in Livingstone's London Mayor electoral campaign, following his admission of using private healthcare after continuously opposing the privatisation of the NHS.
This is not the first time Sugar has criticised the party.
In March Sugar accused MPs of "jumping on the bandwagon of bank bashing", following Miliband's decision to condemn moves to award RBS chief executive Stephen Hester a bonus of almost £1m.