The EU operation is the first of its kind to be commanded by a Briton
European Union warships and aircraft are due to launch anti-piracy patrols off the Somali coast.
At least four ships and two observation planes from several EU countries, including the UK, France and Greece, will escort aid and merchant ships.
The area, including one of the world's busiest shipping lanes, the Gulf of Aden, has seen almost 100 pirate attacks this year.
It is the first naval operation in the bloc's history.
It will take over from the existing Nato mission and last at least a year.
The goal is not only to increase security for seafarers and shipping in the hazardous Somali waters, but to help in the delivery of food aid to Somalia.
EU foreign ministers meeting in Brussels on Monday will formally launch the mission, and approve what officials describe as robust rules of engagement.
But while EU foreign ministers will authorise the use of military force, the fate of any captured pirates will be unclear.
"The EU is reviewing agreements whereby suspects could be taken by third countries that are willing and in a position to launch criminal proceedings," German Foreign Minister Frank-Walter Steinmeier was quoted as saying by AFP news agency.
The operation is the first EU defence mission commanded by a Briton, Rear Admiral Phillip Jones, from the UK's RAF Northwood, which oversees military operations abroad.
Greek Commodore Antonios Papaioannou will run the operation in theatre.
The Saudi supertanker Sirius Star and its crew remain under pirate control after being hijacked in mid-November.
It was the biggest tanker ever hijacked, carrying a cargo of two million barrels of oil - a quarter of Saudi Arabia's daily output and worth more than $100m.
Among other vessels being held is the Ukrainian-owned MV Faina, carrying 33 tanks, as well as grenade launchers and ammunition. It was seized on 24 September.
Various countries have ships patrolling the Somali coast, including the US, India, Malaysia and Russia.