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Up to half a million refugees and other migrants are massing in Libya to ride death boats to Europe.

The warning was given by the captain of HMS Bulwark, which is leading the British mission to save desperate ­passengers of unseaworthy vessels which have killed thousands.

The amphibious assault ship has already helped find around 4,000 refugees before they drowned.

The huge number still planning to make the journey are packed along Libya’s coast.

Captain Nick Cooke-Priest said: “Indications are that there are from 450,000 to 500,000 migrants in Libya who are waiting at the border.”

As he spoke there were reports of another 3,000 migrants in the water.

Britain is working with the Italian navy on a mission codenamed Operation Weald to rescue migrants who have fallen prey to the ruthless people-­smuggling gangs based in Libya.

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Chillingly, occupants of some packed boats have been told to scupper them if they see a rescue ship to make sure they will be saved, though many cannot swim.

Commodore Martin Connell, commanding the Navy operation, said: “This is the most grim ­situation you could possibly find.

"If I could get my hands on some of these smuggling gangs… they have no regard for human life.”

Defence Secretary Michael Fallon was flown on to the ship on one of its two helicopters which scour the seas for the boats.

He urged the rest of Europe to help more as Italy and Britain are bearing the brunt.

Asked about the half a million preparing to make the hazardous journey, Mr Fallon said: “Well you have seen today 3,000 people trying to cross in a single day, we could see hundreds of thousands trying to cross this summer.

“We all have an interest in tackling this much further back.

“The issue here is poverty and conflict in West Africa and poverty and conflict in East Africa. . . .some from Syria. . .more than a dozen countries in west and east Africa.”

He said more work is needed to smash the ruthless people smuggling gangs who con migrants into paying fortunes to make the trip across treacherous waters.

Mr Fallon added: “We can pool intelligence, get after the gangs themselves, we can try to cut off their financing.

"People are making money out of misery and we can do more there to track down the money.

“The Royal Navy stepped up to the plate straight away as David Cameron promised at the emergency summit so we are doing our bit with Bulwark and the helicopters obviously we want other European countries to do more as well.”

So far this year 76,000 migrants have made the 260-mile crossing from Libya to Europe.