The president of one of Britain’s biggest unions has vowed to protect the safety of its female workers, after an independent investigation was launched into the personal conduct of its former general secretary. Barbara Plant, president of the GMB union, said the safety of “our people, particularly women” was paramount to her, following allegations made against Tim Roache. Roache, 57, unexpectedly stepped down from the top role in Britain’s third-largest trade union on Tuesday, after being re-elected just five months ago. Initially the GMB, which represents 600,000 workers including ambulance drivers and manufacturing staff, said Roache had taken the “difficult decision” to resign because of ill health. A day later it updated its statement, giving more details about Roache’s health and confirming that an inquiry into claims made against him was under way. It confirmed it had received an anonymous letter a week earlier. Plant and the GMB’s acting general secretary, John Phillips, wrote to all its staff on Friday afternoon thanking them for their resilience and saying it had “been an incredibly difficult few days for everyone”. The letter said “a full, transparent and independent investigation into the number of very serious allegations made by way of an anonymous letter” had been agreed. It added: “In the meantime, the safety and security of all staff is of paramount importance. We are acting with urgency to create a safe space for staff to anonymously raise … [any issues].” They encouraged people to come forward to report any concerns, and promised their full support. On Thursday Plant had tweeted: “The safety of our people, particularly women, and the integrity of our union is paramount to me as GMB national president. I support a fully independent investigation into the serious allegations raised around the departure of our previous GS [general secretary]. “I support a safe space for our people to come forward with any concerns. We stand for dignity or we stand for nothing.” The Guardian has seen an anonymous letter, which purports to be from “concerned GMB staff”, which urged the union “to suspend Tim Roache with immediate effect, pending a full investigation”. On Wednesday, following questions regarding Roache’s conduct, the GMB issued a statement, which 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.” The statement said Roache – who led the union from 2015 – had been diagnosed some years ago with ME, or chronic fatigue syndrome, and had been advised to refrain from work. Roache, who was often seen as a critic of Jeremy Corbyn, has been a key figure within the union movement. The GMB – which backed Lisa Nandy in the Labour leadership contest – has donated more than £12m to the party over the past five years, according to Electoral Commission records. Earlier on Wednesday, a spokesman for Keir Starmer declined an offer to back Roache following his resignation.