RFA Mounts Bay is one of the fleet's support ships (Picture: Royal Navy).
Almost 700 Royal Fleet Auxiliary (RFA) workers are protesting over an ongoing pay dispute.
Members of the Rail, Maritime and Transport (RMT) union at the RFA are taking action including only working their contracted hours and not preparing food for any social functions such as cocktail parties and social gatherings for ships coming out of refit.
The union will also stage a protest outside the Houses of Parliament, as well as briefing MPs about its opposition to an imposed 1.5% pay rise.
The RMT claimed the "dead hand of the Government" is behind the pay cap.
The union said it is preventing employers from negotiating a settlement which protects RFA workers' standards of living.
Mick Cash, the RMT's general secretary, said: "It is disgraceful that our Royal Fleet Auxiliary members, staff delivering the supplies lifeline to the Royal Navy that is crucial to our national safety and security, have had a derisory pay increase imposed on them which doesn't even keep pace with the cost of living.
"That amounts to a pay cut in real terms and is wholly unacceptable and that is why we have been forced to move to a programme of industrial action."
Watch: What does the RFA do?
Before the first industrial action on 2 July, a Ministry of Defence's spokesperson said: “We are aware of the RMT’s decision to undertake industrial action and are assessing their proposals so we can take appropriate measures to ensure operations continue during this period.
"The pay rise for Royal Fleet Auxiliary personnel for 2018/19 was set in accordance with Treasury guidance.”
Workers taking part in humanitarian operations or reacting to a military conflict are exempt from the industrial action, the RMT said.
The RFA supports and supplies the Royal Navy, providing logistical and operational help.
It is a uniformed civilian branch of the Naval Service, staffed by merchant sailors.