Shadow Chancellor John McDonnell says “a soft coup” has been launched against Jeremy Corbyn by “elements in the Labour Party”.
Mr McDonnell accuses unnamed “plotters” and the “Murdoch media empire” of a “coordinated and fully resourced” coup.
In an article for Labour Briefing, he said “the plotters” were using “an exceptionally well resourced dark arts operation” to “destroy Jeremy Corbyn”.
One former Labour minister described Mr McDonnell’s comments as “unhinged”.
In his article, Mr McDonnell accuses “the coup perpetrators” of “a covert strategy” to destabilise Mr Corbyn because they had failed to dislodge him directly through a leadership election.
He said their tactics included “daily and constant behind-the-scenes non-attributable briefings against Jeremy”.
It is understood the article by Mr McDonnell was written before Labour’s Copeland by-election loss but posted on the Labour Briefing website on Sunday night.
A source close to Mr McDonnell said: “This article was written over a week ago in response to the intervention from Tony Blair.
“It was published in print last week and only went online last night.
“However, as John said yesterday, he wants us all to focus on party unity following last week’s by-election results.
“And he is looking to reach out in the coming days to those across all sections of the party and none.”
‘Scale of task’
Mr Corbyn, from the left of the party, was the overwhelming winner of Labour’s leadership election in 2015, despite having the support of only a small number of the party’s MPs.
A challenge to him last year led to another vote of all Labour Party members and affiliated supporters, which he again won overwhelmingly.
But many of the party’s MPs have continued to be unhappy with his leadership, and the party trails the Conservatives in opinion polls.
On Sunday, Mr Corbyn said Labour’s by-election loss showed “the scale of how hard our task is to persuade people of our message”.
In a speech, he urged his party to “remain united” and not to “give up”.
Labour lost the seat of Copeland, in Cumbria, to the Conservatives last week – the first by-election gain by a governing party in 35 years.
However, Labour managed to hold off a challenge from UKIP in a by-election, in Stoke-on-Trent Central, on Thursday.
Questioned about his future after the speech, Mr Corbyn said: “I’m carrying on as leader because I’m determined that we will deliver social justice in this country.”
Asked if he would still be in place in 2020, when the next general election is expected to take place, he replied: “I’ve given you a very, very clear answer – yes.”