Jeremy Corbyn has insisted he is "very happy" and Labour is moving forward, as a key ally warned a "soft coup" was under way against his leadership of the party.
Shadow chancellor John McDonnell said elements within the party, in alliance with the "Murdoch media", were intent on "destroying" Mr Corbyn and all he stood for.
Mr Corbyn insisted the party was moving forwards under his leadership and would "learn the lessons" of the historic by-election defeat in Copeland, which saw a Labour seat snatched by the Tories.
Labour's Gareth Snell won the Stoke-on-Trent Central by-election, seeing off the challenge of Ukip leader Paul Nuttall, but the party lost Copeland, an area it had held since 1935.
Welcoming Mr Snell to Westminster, Mr Corbyn said his victory showed the people of Stoke "rejecting the politics of fear and instead voting for the politics of hope".
He added: "There was obviously disappointment in Copeland, we will learn the lessons from that disappointment and we will win Copeland back."
Asked about Mr McDonnell's warning of a coup, Mr Corbyn said: "I am very happy, we are going forward. Policies will be coming out, campaigns are going on."
In an article posted on the Labour Briefing website, Mr McDonnell accused the "covert coup plotters" of doing all they could to undermine Mr Corbyn through "an exceptionally well resourced 'dark arts' operation of the old spin school".
He said the plotters, who he did not name, were so determined to get rid of Mr Corbyn they were not only prepared to jeopardise their own seats, they were endangering the very existence of the party.
"We have to alert party members and supporters that the soft coup is under way," he wrote.
"It's planned, co-ordinated and fully resourced.
"It is being perpetrated by an alliance between elements in the Labour Party and the Murdoch media empire, both intent on destroying Jeremy Corbyn and all that he stands for."
Mr McDonnell's warning comes after Labour former cabinet minister Lord Mandelson disclosed last week he was working "every single day" to bring Mr Corbyn's leadership to an end.
Mr McDonnell said, after last year's failed coup attempt, the plotters realised a direct attack on Mr Corbyn was liable to provoke a backlash from grassroots party members, so they were now working behind the scenes to destabilise him.
Their tactics included "daily and constant" behind-the-scenes briefing against Mr Corbyn and the shadow cabinet every time one of them made a statement, intervened in Parliament or launched a new policy.
"The plotters use every opportunity to chip away at Jeremy's standing to seek to demean him and undermine support for him in the Labour Party and among Labour supporters," he said.
"This constant barrage of negative briefings also crowds out any positive initiatives or narrative from Jeremy and his team.
"It also feeds and confirms in the public's mind that the Labour Party is split.
"The professional planning of interventions in which attacks to undermine Jeremy are framed evidences an exceptionally well resourced 'dark arts' operation of the old spin school."
The shadow chancellor said the plotters were effective in distorting the media coverage because they had "extensive contacts and allies" in the media, many inherited from Lord Mandelson's days when he was part of the leadership.
He said it had led to the appearance of "fake news" stories in The Times and The Sun claiming Mr Corbyn was about to stand down as leader, which were then taken up by other media outlets.
"The coup plotters are willing to sacrifice the party at elections just to topple Jeremy and prevent a socialist leading the party.
"It is more important to them that they regain control of the party than it is to win elections," he said.
"The irony is that they are willing to go so far in denigrating Jeremy that they endanger their own parliamentary seats and endanger the very existence of the party they want to use to get into power."
A spokesman for Mr McDonnell said the article had been written and published in print last week in response to Tony Blair's attack on Labour over Brexit, although it only appeared online on Sunday.
"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," the spokesman said.
Mr Corbyn was in the Commons chamber to see Mr Snell sworn in on Monday afternoon.
Mr Snell was walked to the despatch box by his colleagues Ruth Smeeth and Jack Dromey, amid cheers from the Labour benches.
The new MP chose to make a solemn affirmation instead of swearing an oath.
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