A bug that causes a chronic wasting disease in cattle could be passed to humans through milk, it was claimed today.

As many as three pints in every hundred could be contaminated with the Johnes disease bacteria, it is alleged.

Medical experts are claiming a link between Johnes disease in cattle and the incurable Crohn's disease, which causes chronic inflammation of the bowel.

Johnes disease affects cattle in the same way that Crohn's afflicts up to 80,000 people in Britain.

It is believed to infect up to a quarter of dairy cows in many developed countries.

Professor John Hermon-Taylor, head of surgery at St George's Hospital in London, believes the same bacterium - Mycobacterium Avium subspecies Paratuberculosis, or MAP - is responsible for both diseases.

"MAP is widely prevalent in the dairy cattle of western Europe and the United States and the human population are almost certainly exposed to it," said Professor Hermon-Taylor.

He outlines his fear that the bug is being transmitted to humans through milk in an investigation by the BBC's Rural Affairs Unit to be broadcast this weekend.

Professor Hermon-Taylor is supported by Professor Michael Collins, from the International Association for Paratuberculosis.

Professor Hermon-Taylor said: "We carried out an exhaustive four and a half year study with funding from the Ministry of Agriculture and it showed that MAP was present in retail milk."

He said the study led in 1998 to a voluntary increase in milk pasteurisation time from 15 to 25 seconds, aimed at killing off the bug.

But Professor Hermon-Taylor said a continuing study for the Ministry of Agriculture at Queen's University in Belfast had shown that MAP was still present in retail milk.

Despite the more stringent safety precautions, 3% of milk sold for human consumption contained the bacteria, it was claimed.

The BBC said that after a cluster of Crohn's cases occurred in Cardiff, a study of the city's River Taff had also revealed the bug in water droplets. It was feared that MAP could be washed off fields into water courses.

Professor Roger Pickup, from the Institute of Freshwater Ecology at Lake Windermere, who was called in to investigate the outbreak, believes MAP may also be infecting the human population through tap water.

"Our theory is that when the river flows through the city, tiny droplets of water containing MAP are blown by the wind and breathed in by the population," he said.

Dr Norman Simmonds, the Government's Chief Scientific Adviser, told the BBC: "The evidence is not enough to make us change the precautions we already take, or make us do more than we are doing - which is to gather more scientific evidence."

The reports were broadcast on Radio 4's Today programme today and will also be in BBC 1's Countryfile programme on Sunday.