Last week Bulwark plucked 747 people from dangerously overcrowded boats in a similar operation and it is thought yesterday’s rescue will take the number of lives saved to more than 2,000 since it began its work there last month.
All European naval ships in the region were ordered to go to the area after intelligence suggested up to 14 migrant vessels were on the open water.
A Ministry of Defence spokeswoman said: “From first light this morning, a Merlin helicopter from 814 Squadron on board HMS Bulwark has been conducting surveillance operations and has identified four migrant vessels in distress, containing a total of at least 500 people.
“HMS Bulwark has now commenced rescue operations to recover those in the vessels.”
The 19,000-tonne assault ship was sent by the Government on May 4 to help search and rescue efforts in the Mediterranean in a bid to tackle a dramatic increase in the number of people dying trying to cross the sea.
It is estimated that more than 1,600 people 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.
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, successfully located 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 it successfully located and rescued a barge with about 310 people on board, approximately 30 miles north of the city of Zuwarah, Libya, after they had initiated a distress call.
Weather conditions at the time were difficult, with dense fog hampering the operation.
The people saved were 212 men, 59 women and 39 minors.
This was the sixth successful operation the ship has undertaken since leaving Cork, bringing the total of migrants saved since commencing operations to more than 1,000. It has now resumed patrol and is expecting further operations in the coming hours.
Bulwark is leading Britain’s contribution to the operation European leaders hope will stem the loss of life of refugees fleeing turmoil in Africa and the Middle East.
Bulwark, a landing platform dock, is designed to put ashore Royal Marine commandos in assaults by sea, by boats launched from the dock compartment, and from two helicopters from the deck.
It has a nautical range of 8,000 miles and can carry up to 700 troops on top of a crew of 325.