By Red Harrison BBC Sydney Correspondent

Scientists in Australia believe what was thought to be a new species of poisonous jellyfish, which has killed two tourists in recent weeks, has in fact been killing swimmers off the Great Barrier Reef for years.

Irukandji jellyfish: The size of a peanut

The two dead tourists, a Briton and an American, were believed to have been the first to be linked to a tiny and mysterious species of jellyfish called irukandji.

Both men showed symptoms of having suffered a stroke or heart attack, and both fell unconscious within half an hour of being carried from the water.

But in both cases, jellyfish were captured with their victims, enabling doctors to confirm they had been stung with a venom for which no antidote is known.

Heart attack cases

Robert King, a 44-year-old from Columbus, Ohio, died on Friday when the sting brought on a rapid rise in his heart rate and blood pressure, leading to a cerebral haemorrhage. He was stung while he was diving on the Great Barrier Reef off Port Douglas, Queensland, late last month.

In January, British tourist Richard Jordan, 58, from Driffield in north-east Yorkshire, became the first recorded fatality from an irukandji sting, after being stung while swimming off Hamilton Island in Australia's north.

Among scientists researching the irukandji, Doctor Peter Fenner, chief medical officer for Surf Life Saving Australia, says he has seen identical symptoms among surfers in North Queensland for more than 30 years.

He suspects many reported to have had a stroke or heart attack have in fact received a lethal sting from the irukandji.

One problem in finding and catching specimens of this particular jellyfish is that some are no more than 12 millimetres across.

Doctor Fenner believes many more new species and varieties of jellyfish remain to be discovered and there is little doubt, he says, many will carry lethal poison.