Bracken, the familiar green fern, is "out of control", will be particularly dangerous this year, and holidaymakers should avoid it like the proverbial plague, they say.
It sounds like the first scare of the silly season, to rank with such well-worn shock-horrors as the perennial warnings of invasions of killer bees and jellyfish. But the threat from the plant is indeed serious. Bracken can cause cancer, can pollute milk and drinking water, harbours disease- bearing ticks, and costs farmers millions of pounds each year. But government spending cuts and European Union rules designed to prevent the pollution of drinking water are making it more difficult to stop its spread.
The distinguished group are joining forces in the official-sounding Bracken Advisory Commission - actually a voluntary body whose name reflects the growing tendency among environmental groups to give themselves portentous titles - led by Professor Jim Taylor, emeritus professor of geography at the University of Wales, Aberyswyth.
Bracken (Pteridum aquilinium) now covers an area the size of Yorkshire and is increasing its coverage of some parts of Britain by 3 per cent a year. Britain has 70 per cent of the world's heather but a quarter of this has already been lost to the plant and another 45 per cent is at risk.
Worse, adds the commission: "The plant, which many view as part of the countryside's natural beauty, is actually extremely poisonous to wildlife, livestock and humans. It has the capacity to kill animals which eat it, and poses a serious threat to people who walk in it. Literally thousands of people, many with their children and pets, could be put at risk of contracting serious illnesses."
The plant contains cancer-causing chemicals related to benzene. Scientists who fed mice and guinea-pigs on the spores released by bracken found they developed cancer, says the group.
Japan, where young bracken is eaten as a delicacy, "has one of the highest rates of stomach cancer in the world". In Costa Rica "there is established evidence of human cancers caused by the drinking of milk from cattle which have eaten the plant".
Professor Taylor says: "It is yet to be proved beyond doubt that cancers in humans can be caused either by the inhalation of bracken spores, the drinking of untreated milk or water, or the eating of parts of the plant itself. However, it can only be a matter of time before such evidence is gathered."
The group says that these spores release cancer-causing chemicals in water and fears they will contaminate water supplies - but adds that EU rules frustrate attempts to prevent this.
It says the herbicide used to attack bracken is perfectly safe and very effective, but that EU limits for any pesticide in drinking water are so strict that no spraying can take place within 160 metres of a watercourse. Meanwhile, the Government this year eliminated grants to farmers to help them fight the plant over much of England.
The group adds that bracken harbours ticks which can give people Lyme Disease, which can affect the central nervous system, while every year "livestock losses in sheep and cattle due to bracken-related problems are estimated to top pounds 8m".
Another commissioner, Professor Roy Brown - who ran the North York Moors National Park for 15 years and is now director of research at Bishop Burton College, part of the University of Lincolnshire and Humberside - says that this year's dry weather is expected to produce an unusually high number of bracken spores over the next weeks. He says: "We will be advising people not to go in or near bracken from now on." In 1989, the last time there was a heavy sporing season, shepherds and farm workers were advised to wear masks in bracken.
The commission is hoping to be registered as a charity and to raise money from organisations that control bracken, including the firms that make the pesticide and spraying equipment. The commissioners are not making anything out of it: "We're doing it for love, if that's the word," says Professor Brown.