The leader of Britain’s biggest trade union has attacked a decision by Tom Watson to call off a meeting between Jeremy Corbyn supporters and rebelling MPs as an “act of sabotage” against the Labour party.

Len McCluskey, general secretary of Unite, said a workable plan to resolve bitter differences between the Labour leader and the parliamentary Labour party had “never been closer”.

Watson, the deputy leader, announced that a planned meeting on Sunday with McCluskey and other leading Labour figures would not go ahead, prompting a leadership challenge from Angela Eagle and a vow from Corbyn to fight any moves against his position.

The Labour leader’s spokesman said it was disappointing that Watson had walked away from the union-brokered talks, which were attempting to end the impasse between the embattled Corbyn and his MPs.

Tom Watson, Jeremy Corbyn and Angela Eagle during a referendum event in London. Photograph: Frank Augstein/AP

Eagle then announced her intention to fight, saying Corbyn had failed “to lead an organised and effective” opposition to the Conservative government.

But in a sign of the deepening divisions between factions in the Labour party, McCluskey described Watson’s announcement as a “deeply disingenuous manoeuvre”.

“I am dismayed at the statement issued by Tom Watson announcing his withdrawal from talks aimed at resolving the crisis in the Labour party,” he said.

“Extraordinarily, I received no notice of this statement before it was issued. I had made arrangements for a meeting of trade union leaders, Tom Watson and representatives of the PLP and the party leader for tomorrow, arrangements requested by Tom Watson and his colleagues, specifically for Mr Watson’s convenience.

“In that context, when the possibility of a workable plan had never seemed closer, Tom Watson’s actions today can only look like an act of sabotage fraught with peril for the future of the Labour party.”

He added: “I must clarify one point in Tom Watson’s statement; I made it absolutely clear from the outset of these discussions that Jeremy Corbyn’s resignation as the leader was not on the agenda. Watson knew that, and it is entirely wrong to suggest that any public statement by Jeremy represented any change in the situation. This is a deeply disingenuous manoeuvre.

“I will continue to work with trade union colleagues and others to chart a way forward, including meeting the legitimate concerns of Labour MPs.”

The Unite leader suggested Eagle, or any other candidate to replace Corbyn, was unlikely to win the support of any of the party’s influential trade union backers.

“Should there have to be a leadership election, I must warn that any attempts to keep Jeremy Corbyn, elected just 10 months ago with an enormous mandate, off the ballot paper by legal means risks a lasting division in the party,” McCluskey said.

“It is time for everyone to commit to a democratic and dignified procedure as the only way to avert such a disaster for working people.”