HMS Bulwark is involved in a rescue mission to save at least 500 migrants found in four boats in the seas off Libya.
The Royal Navy warship has returned for another operation after plucking 747 people from dangerously overcrowded boats in waters off the north African country last week.
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 onboard 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 to help with 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.
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.