More than one-quarter of rescue co-ordinator posts at Aberdeen coastguard station are unfilled this summer, and Shetland station is missing one-third of its staff, claimed the Public and Commercial Services Union (PCS).
It said Maritime and Coastguard Agency (MCA) figures show Aberdeen is seven staff short of its full complement of 26 and Shetland is six down on the 18 it should have.
Last month the union also revealed almost half of the shifts at Aberdeen were staffed below the safe risk-assessed levels for more than a year. Between May and September 2012 this rose to 54%, it said.
Across the UK, almost one-quarter of all watches were staffed below the risk levels, and four others - Belfast, Dover, Falmouth, and Thames, in Essex - were staffed below safe levels more than one-third of the time.
The union blames the uncertainty caused by the Government pressing ahead with the closure of nine of the UK's 19 coastguard stations and cutting 140 jobs.
The Clyde and Forth stations, as well as Yarmouth, have already been shut.
PCS general secretary Mark Serwotka said: "Officers have been leaving in droves because of the Government's ill-thought-through closure plans and ministers and senior officials need to address this urgently."
Aberdeen-based Steve Quinn, president of the MCA section within the union, said the shortages put a lot more pressure on those who remained in post, which was bound to put lives at risk. The MCA said it had recently completed a recruitment campaign where 28 watch officer vacancies were filled at locations around the coast, and another campaign was under way.
It said: "Currently, where a Maritime Rescue Co-ordination Centre (MRCC) is experiencing reduced staffing levels, established 'pairing' arrangements are used. This means each MRCC can be connected to at least one other MRCC that will provide mutual support."