Up to half a million refugees are massing in Libya to try and flee to Europe on the death boats that have killed thousands already.

The shocking toll of misery was revealed yesterday by senior Royal Navy officers leading Britain's Mediterranean rescue mission off the Libyan coast.

Britain's amphibious assault ship HMS Bulwark has helped find around 4,000 refugees before they drowned in hopelessly unseaworthy boats.

The ship's 350-strong company of sailors and Royal Marines is bracing itself to steam to the rescue of a further 3,000 risking death to flee war torn countries and poverty in Africa to attempt the perilous crossing.

Bulwark's 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."

Even as he spoke intelligence reports were warning Bulwark's crew - which is among around 11 rescue vessels in the central Mediterranean - of another 3,000 migrants in the water.

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

Chillingly the occupants of some of the overcrowded boats are thought to have been told to scupper them as soon as they see a rescue ship - to ensure they will be rescued, even though many of them cannot swim.

Commodore Martin Connell, the Royal Navy's senior officer commanding Operation Weald, is on board to check on operations.

"This is just the most grim situation you could possibly find," he said. "It is a grim business this ... most of these boats won't make it.

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

Defence Secretary Michael Fallon visited Bulwark, flown in on board one of the ship's two Mark II Merlin helicopters, which are used to scour the seas for stranded, overcrowded boats.

He urged the rest of Europe to pitch in with more help as Italy and Britain are taking on the brunt of the rescue workload.

Asked about the half a million already preparing to make the hazardous journey by boat from Libya he told reporters: "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 into Europe from Libya, with 40,000 remaining in Italy.

Yesterday Bulwark turned dramatically into the wind as once more it launched a Merlin helicopter to investigate reports of thousands of migrants who needed to be rescued.