Tim Roache, the general secretary of the GMB trade union, has abruptly resigned just five months after being re-elected.

In a statement released late on Tuesday night, the union, which represents more than 600,000 workers and has more than 80 members among MPs, blamed ill health for his sudden departure.

“Tim has unfortunately been suffering with ill health for some time now and has made the difficult decision to stand down from his role, that he does not feel able to continue, in leading our union going forward,” it said.

However, some union sources suggested Roache’s departure followed a clash with senior GMB officials over allegations relating to his personal conduct.

The statement continued: “A meeting with the finance and general purposes committee will be convened to discuss the resignation and further updates will follow.”

The GMB backed Lisa Nandy in Labour’s recent leadership contest, helping her to secure a place on the final ballot paper, and Angela Rayner for deputy leader.

The union holds two seats on Labour’s ruling national executive committee.

Roache is a GMB lifer, having started in the union’s postroom in 1979, and rising to be its general secretary in 2015 – being regarded at the time as from the leftwing of the Labour movement. He was re-elected last November with 61% of the vote.

During Jeremy Corbyn’s leadership, the GMB was sometimes regarded as a more moderate voice than Unite, whose general secretary, Len McCluskey, was an outspoken backer of Corbyn.

Roache spoke out about the party’s handling of antisemitism in 2018, saying he had been shocked by some of the examples he had seen.

“Antisemitism is a problem in the Labour party. I’ve been shocked at what I’ve seen on social media,” he said.

More recently, he hit back on social media at critics of the GMB’s response to the leaked Labour report, which reproduced dozens of lurid private WhatsApp messages from staff hostile to Corbyn.

The GMB branch had called for Labour to protect those named in the report. Replying to claims that this was an attack on the party’s general secretary, Jennie Formby, Roache tweeted: “The GMB is NOT going for Jennie. FACT.”