The United Voices of the World (UVW) union, whose members are striking for 48 hours at St George’s Hospital, University of London, are taking legal action after its lawyer was arrested on the picket line.
Police also threatened the 25 workers, security guards employed by contractor Noonan, who are paid less and suffer worse working conditions than directly employed staff at the university.
The union is also demanding equal treatment over pay, holidays and pensions.
On 13 January police attended the picket line and attempted to remove the strikers from NHS grounds, arresting Franck Magennis, the head of UVW’s legal department.
UVW say: “This could set a dangerous precedent for the erosion of workers’ rights if left unchallenged.”