The MP, who did wish to be named, told The Yorkshire Post that Pudsey candidate Jane Aitchison was “ultra-left” and would not get the backing of voters in the super-marginal Leeds seat held by Conservative Stuart Andrew with a majority of just 331.
Ms Aitchison has been forced to apologise for the language she used in a string of controversial Twitter posts, and has retained the backing of the Labour Party.
The candidate, who was backed by Unite and Momentum as well as key Jeremy Corbyn allies John McDonnell and Jon Trickett, in December 2013 posted a link to an article about a “middle-class revolt” over free coffee at the supermarket Waitrose, adding: “From the comments seems real scum already shop there”.
In another tweet a month later, she appeared to post an article about fast-food chain McDonald’s creating 2,500 jobs, adding: “S**t food. S**t jobs. This is what they want for us.”
And in a series of tweets she took aim at the Royal Family. As reported by The Yorkshire Post on Monday, Ms Aitchison deleted a tweet describing Prince Charles as “benefit scrounging scum”.
But in another undeleted post unearthed by the Guido Fawkes political blog, she comments on an article about Buckingham Palace seeking a housekeeper: “I am sick of paying for these lazy scroungers.”
Ms Aitchison, who in 2011 left the Socialist Party, formerly the Trotskyist Labour entryist group Militant, praised attendees at the Socialist Workers Party-hosted Marxism Festival of July 2015 as “the best class fighters”.
Responding to her selection, the unnamed Labour MP said: “We might as well kiss that seat goodbye for Labour. I know the electorate in Pudsey will take a different view to the selectorate in Pudsey Labour Party.
“I don’t think the party members in Pudsey are representative of the community.”
The MP added: “She’s ultra-left, once people know what that means they won’t vote for it.
“People from Leeds aren’t radical revolutionary socialists, some of them are Labour, some of them are Conservatives but they’re not revolutionaries and they don’t believe in revolutionary politics.”
Ms Aitchison later apologised, and said: “I recognise that the language used in some tweets I made years ago is unbecoming of a parliamentary candidate and I apologise.
“I’m thrilled to be selected as Labour’s candidate in this important seat and I am putting forward Labour’s positive message of hope and optimism here, with the aim of becoming the MP for the many not the few.”
Meanwhile, concerns were raised about the selection process as some members felt debate about Ms Aitchison was shut down in a Facebook group for the constituency party which banned posts that “disparaged” any of the candidates.
One active Labour member in Pudsey, who did not want to be named, said: “There was a real attempt in the constituency to shut down any discussion about the candidates which could be construed as being negative.”
A Labour Party spokesman said: “Jane Aitchison has been democratically selected by members in Pudsey in line with Labour’s rules and procedures.
“Jane is looking forward to putting forward Labour’s message of hope.”