They will join more than 130,000 civil service workers in staging strike action on Friday 28 April in an ongoing dispute with the government over pay, pensions and job security.
It follows action over Christmas and in the lead up to new year, as well as several subsequent walkouts at ports across the UK and in northern France.
The PCS union, which represents around more than 130,000 civil service and public sector workers, said a month of action beginning 3 April "to increase pressure on government" would culminate in the all-out strike on 28 April.
