The Royal College of Nursing has cancelled its annual meeting of members in Liverpool following "serious allegations of sexual harassment".
The union, which has 465,000 members, said the three-day congress in September would now be held online after "independent legal advice".
It said that, "as a matter of urgency" it was reviewing "all safeguarding policies and procedures".
The RCN did not provide any further details of the allegations.
In a statement, the RCN said its elected council "decided the best way to safeguard members attending congress is to move this year's meeting to a fully virtual event".
Carol Popplestone, the council chair, said: "We have acted fast and took a unanimous decision that protects all attendees.
"We know that many members planning to attend were looking forward to seeing each other in person for the first time in two years.
"But this decision was made with members' safety at the top of our minds. That is what we are here to safeguard at all costs.
"I am absolutely determined that congress will still be its usual fascinating mix of debates, events and speeches. Getting together virtually won't stand in the way of that."
The annual gathering was previously planned to be held both in-person in Liverpool and virtually due to the coronavirus
pandemic - last year's event did not take place due to restrictions.
The 2019 congress took place in Liverpool from 19 to 23 May.
The RCN describes itself as the world's largest nursing union, representing registered nurses, student nurses, midwives and nursing support workers across the UK and internationally.