Labour leader Jeremy Corbyn is under fire after publicly backing a top policy adviser after he was suspended over complaints he campaigned for a rival party at the general election.
Mr Corbyn insists he has "full confidence" in Andrew Fisher even though he faces possible disciplinary action by the party's ruling National Executive Committee (NEC).
However former Parliamentary Labour Party chair Lord Soley warned that by bringing in such a divisive figure, Mr Corbyn was simply playing in to the hands of the Conservatives.
"We don't want people who have been slagging off Labour candidates and telling them not to vote Labour," he told the BBC Radio 4 Today programme.
"What I am angry about is we are betraying the very people who voted Labour in the hope they would get a Labour government. Going on like this, we will continue to keep the Conservatives in power.
"This is very similar to the position the Tories got themselves into when they had Iain Duncan Smith as leader and we actually did all we could to keep him there. The Tories are doing exactly the same with Jeremy now."
But former London mayor Ken Livingstone dismissed the complaint against Mr Fisher as a "complete nonsense" and accused Blairite MPs of trying to undermine Mr Corbyn's leadership.
"The MPs who have taken this up and the people driving this aren't really terribly concerned with this one individual, they are trying to undermine the leader who has just been elected and that's completely unacceptable," he told the Today programme.
"If you are one of those New Labour MPs who thinks that the Blair government was the apex of human civilisation you have got to come to terms with the fact that the party has moved on."
A number of Labour activists and MPs are understood to have made complaints over allegations that Mr Fisher urged supporters to vote for a Class War candidate against Labour's Emily Benn in the Croydon South seat in the May election.
Ms Benn - the granddaughter of Labour's former Cabinet minister Tony Benn - lodged a formal complaint about Mr Fisher after he sent a tweet in August last year, which read "FFS if you live in Croydon South, vote with dignity, vote @campaignbeard" - a reference to the Twitter account of Class War parliamentary candidate Jon Bigger.
Labour's rule book states that any member who "supports any candidate who stands against an official Labour candidate ... shall automatically be ineligible to be or remain a party member".
Mr Fisher last week wrote to Labour's general secretary Iain McNicol to "completely and unreservedly" apologise for the tweet, which he claimed had been "misinterpreted".
The message about Croydon South was not the only controversial social media post made by Mr Fisher, who also described Ed Miliband's frontbench as "the most abject collection of complete s***e" and celebrated Ed Balls's general election defeat.