British troops are returning from Afghanistan on HMS Albion
HMS Albion has left the Spanish port of Santander with 450 service personnel who were stranded due to the volcanic ash cloud grounding flights.
The Royal Navy ship is bringing home soldiers, RAF personnel, medics and support staff who have been deployed in Afghanistan for six months.
HMS Albion has also rescued about 280 "vulnerable" British nationals.
The ship left Santander at about 1230 BST and is expected to arrive back in Portsmouth late on Wednesday.
'Poised and ready'
The Ministry of Defence said the military personnel includes members of RAF 9 Squadron, 33 Medical Regiment, the Royal Anglian Regiment and battle group support staff.
An MoD spokesperson said the government was looking at a number of ways to help other British citizens who are stuck abroad.
"HMS Ocean and HMS Ark Royal are poised and ready for tasking if directed," a statement said.
A spokeswoman from the Foreign and Commonwealth Office told BBC News the 200 civilians being brought back were "vulnerable" British nationals.
HMS Ocean is a commando helicopter carrier and the navy's biggest ship
HMS Albion, which acts as a command platform for the Royal Navy's amphibious task force and landing force, is capable of carrying more than 600 personnel as well as vehicles and combat supplies.
Commando helicopter carrier HMS Ocean, the navy's biggest ship, left Plymouth's Devonport Naval Base on Monday morning on a routine training exercise.
Meanwhile, Brittany Ferries told BBC News it has no space on any of its Spanish services until 29 April.
It said people should not to go to Santander as all commercial ferries are full.
Instead it urged foot passengers who wish to sail into Portsmouth to go to the French ports of Caen, Cherbourg, Roscoff and St Malo.