More than 70 medical secretaries at Altnagelvin Hospital today went on strike as anger grew over increased workloads.
Nipsa members had balloted to down tools in a one-day strike, to be immediately followed by work to rule.
Union officials confirmed this will include a ban on overtime and non-cover for vacant posts, which could result in patient's letters not being typed or distributed to GPs, theatre lists not being typed and results delayed.
The union has claimed that management have failed to replace medical secretaries, and that the Western Health and Social Care Trust was not resourcing secretarial and administrative support for the additional medical appointments made at the Derry hospital. The Trust said today they were awaiting funding to refill vacancies.
The secretaries had originally planned to strike last month, but agreed to suspend action in order to provide the WHSCT with a chance to bring formal proposals to alleviate a "staffing crisis".
Kevin McCabe, Nipsa assistant secretary said: "It is not enough for management to acknowledge that there is a shortage of medical secretaries; action is necessary to fill the vacancies."