The Ministry of Defence has said the find could be "the biggest ever".

The British frigate HMS Marlborough was taking part in a routine Anglo-American drug-smuggling operation when it stopped the ships.

It found bales of cocaine on the Panamanian-registered merchant ship China Breeze, some 200 miles south of Puerto Rico on 27 May.

The drugs had been hidden in amongst the cargo, under deck plates and in a sewage tank.

The second bust was made on 31 May on another Panamanian-registered merchant vessel, the Castor, near the island of Isla Margarita.

Possible link

The Ministry of Defence could not confirm whether the two seizures were connected, but did not rule it out.

A spokesman said: "Given they were both Panamanian-registered vessels in the same area within days of each other, that's an assumption that could be made."

Defence Secretary George Robertson said: "One drugs bust of four tonnes of cocaine is good. Two in one week is even better. And we will not give up here. We will continue to catch the people who deal in drugs."

For decades drug barons have used boats and planes to take drugs from South America to Caribbean islands, and from there to the United States.