Coastguards and maritime officials are taking industrial action in a row over a below-inflation pay rise.
The MCA employs about 1,165 staff across the UK
Up to 700 members of the Public and Commercial Services union across the UK are taking part in the action.
The Maritime and Coastguard Agency staff withdrew from non-essential duties such as issuing fishing vessel certificates and incident reports.
Their union said staff felt "betrayed" when the offer was compared to those given to other emergency services.
It said the basic coastguard grade was paid barely above the minimum wage at £5.37 an hour for a 42-hour week.
PCS general secretary Mark Serwotka said the pay rise averaged 2.5% for many staff, while the most experienced workers would get less than 1%.
"Management could have avoided damaging industrial action by negotiating seriously and offering decent pay to those who deliver a vital emergency service," he said.
"Below-inflation pay is completely unacceptable."
The MCA employs about 1,165 staff, supported by more than 3,250 volunteer coastguards.
Its tasks include co-ordinating search and rescue at sea and ensuring ships meet UK and international safety rules.