Last week the British warship saved 747 migrants from dangerously overcrowded boats in a similar operation.
We want to save livesDavid Cameron
The latest mission is thought to bring the number of people rescued by HMS Bulwark to 2,000 since it started operations in the Mediterranean in May.
All European naval ships nearby were ordered to go to the area after intelligence suggested up to 14 migrant vessels were on the open water.
David Cameron said from the G7 summit: “We want to save lives – Britain is a country that doesn’t walk on by, we’re a country with a conscience.
“But we also need to do more to stop these people leaving their countries in the first place – that’s what we’re using our aid budget for.”
He added: “We need to deal with the causes of this immigration, not simply with its consequences.”
HMS Bulwatrk was deployed to help tackle a dramatic increase in the number of people dying trying to cross the Mediterranean to reach Europe.
More than 1,600 people are estimated to have drowned so far this year trying to make the crossing.
Many are fleeing war in Libya, where Islamic State fighters are terrorising the population, fuelling instability in the war-torn country.
‘We need more ships’
Defence Secretary Michael Fallon visited the ship and said: “We could see migration attempted on a colossal scale, hundreds of thousands of people, if Europe does not get together and start tackling the problem much further back.
“In the short term, we need more ships from other European navies to come and help here, we need to pool more intelligence, we need to find out who is doing this trafficking, how they are making money from it, and we need to go back and smash the gangs themselves.”
On Friday an Irish Navy ship, the LE Eithne, found and rescued a small craft with 113 people on board that was in distress 40 km north of Tripoli, Libya.
The men, women and children were transferred to an Italian ship for onward passage to a port of safety.
Yesterday the same ship successfully located and rescued a barge with about 310 people on board, about 30 miles north of the city of Zuwarah, Libya, after they had sent out a distress call.
The ship saved 212 men, 59 women and 39 minors.
Mr Cameron has pledged three of Britain’s Merlin helicopters to help the migrant rescue effort.