Hundreds of coastguards and other maritime workers are due to launch a campaign of industrial action in a row over pay.
Up to 700 members of the Public and Commercial Services union will withdraw from non-essential duties, including the issuing of fishing vessel certificates and production of incident reports.
The Maritime and Coastguard Agency workers are protesting at a pay rise which the union said averaged 2.5% for many staff, falling to 1% to the most experienced employees.
General secretary Mark Serwotka said: "Management could have avoided damaging industrial action by negotiating seriously and offering decent pay to those who deliver a vital emergency service.
"Below-inflation pay is completely unacceptable with the basic coastguard grade paid barely above the minimum wage at £5.37 an hour for a 42-hour week.
"Staff who monitor shipping and public safety across tens of thousands of square miles of ocean and coastline feel betrayed and have been further angered about the total disregard of the pay comparison evidence with other 999 services."
