Left-wing campaign group Momentum has warned it will mobilise to defend Labour leader Jeremy Corbyn if there is any move to unseat him in the wake of the May elections.
Jon Lansman, one Mr Corbyn's closest allies and a key figure in the group which is estimated to have up to 100,000 supporters, said it was "absolutely equipped" to see off any challenge to his leadership.
The warning will alarm some Labour MPs who regard Momentum - formed to capitalise on the enthusiasm generated by Mr Corbyn's successful leadership campaign last summer - as a "party within a party".
It comes amid speculation that critics of Mr Corbyn could mount a leadership challenge if the party performs poorly in elections to the Scottish Parliament and English local authorities due to be held in May.
Mr Lansman said there appeared to be people in the party who were seeking to "create an opportunity" to oust the leader and made it clear that they would be resisted.
"We campaigned to elect him and we will most certainly defend him and support him if there is a challenge," he told The Guardian.
"We absolutely have the machinery to run another leadership campaign if we have to and we are absolutely equipped to do that. And we will if necessary and I am confident we will win it."
He added: "I have to say there are some people in the party who are more interested in creating an opportunity to challenge Jeremy than to win in May and I very much regret that.
"I very much regret that they do not seem to care about creating an appearance of disunity which does damage to the party."