The former head of one of the UK’s biggest unions, who resigned unexpectedly on Tuesday, is at the centre of an internal investigation into his personal conduct. A statement from GMB union on Wednesday said officials have launched an inquiry into anonymous claims made against Tim Roache, who stood down just months after being re-elected as general secretary. The development has left the union, which represents 600,000 workers including ambulance drivers and manufacturing staff, in turmoil. Following a number of questions from the media regarding circulated allegations about Roache’s conduct, the statement said: “GMB received an anonymous letter, last Wednesday, in which a number of allegations have been made about Tim’s conduct whilst he held the office of general secretary. “An investigation has been launched and it is not appropriate to comment further pending the outcome of that investigation. “Complaints which are brought to our attention will be dealt with appropriately in line with our policies and procedures and with appropriate regard to fairness and sensitivity.” The statement said Roache had been diagnosed some years ago with ME, or chronic fatigue syndrome, and had been advised to refrain from work. “Having been signed off by medical professionals earlier this week, Tim was advised to refrain completely from work – and has now taken the difficult decision to stand down from the role permanently, indicating that he does not feel mentally or physically able to continue to lead our union,” it read. The decision seemed to have coincided with a campaign for him to be suspended from office. Sources close to the union said a letter had been sent to the union’s president Barbara Plant which made detailed, personal allegations against Roache. Some claims have also been circulated on social media, the source said. “They are serious claims and are going to be investigated properly,” a source said. Roache wielded considerable influence within the union movement, given that the GMB is one of the biggest donors to the Labour party. It has given more than £12m to the party over the past five years, according to Electoral Commission records. He was often seen as a critic of Jeremy Corbyn. In 2016 the GMB supported Owen Smith in his leadership challenge. This year, the union backed Lisa Nandy, who came third — behind Keir Starmer and Rebecca Long Bailey — and is now the shadow foreign secretary. Roache was first elected as GMB leader in 2015 and was re-elected in 2019. The union said that John Phillips, its regional secretary for Wales and the South West, has been appointed as acting general secretary until a replacement for Roache is elected. Earlier on Wednesday, a spokesman for Keir Starmer declined an offer to back Roache following his resignation. Roache has been approached for comment.