HMS Cutlass has been commissioned into the Royal Navy fleet in a ceremony at Gibraltar Naval Base.
She is the first of two new £5m patrol boats in charge of guarding the waters around the Rock and has already started patrols.
The hoisting of the White Ensign on Cutlass comes after months of sea trials.
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The two agile patrol boats will replace P2000 HMS Purser and her sister HMS Dasher in Gibraltar's waters.
Cutlass and Dagger are 19m long and can hit speeds of 40 knots.
Both boats are equipped with three machine-guns and the latest electronic optical equipment to assist in identifying potential threats.
Lieutenant Commander Adam Colman assumed command of Cutlass in the ceremony, watched by more than 100 sailors and their families.
"It is an honour to be able to mark this very special occasion today," he said.
Gibraltar's Commander British Forces, Commodore Steve Dainton, His Excellency Sir David Steel, Chief Minister Fabian Picardo and Mrs Justine Picardo, who was named the Lady Sponsor of the vessel, were also in attendance.
The Gibraltar Squadron provides security in British Territorial Waters and protection to visiting warships, submarines and support vessels alongside their civilian counterparts in the Gibraltar Defence Police force.