A parasite spread by cats is infecting 1,000 new people every day in Britain – about 350,000 a year – according to an official assessment of the risks posed by toxoplasma, which can cause serious illness and has been tentatively linked with schizophrenia and other psychotic disturbances.

In news that will challenge public perceptions about the country's most popular pet, official figures to be published later this week will reveal the shocking levels of infection within the UK human population of Toxoplasma gondii, a microscopic parasite that forms cysts in the human brain and other vital organs of the body.

Toxoplasma infections come either through direct contact with cats or from eating contaminated meat or vegetables, tests on British blood donors have revealed.

Although the clinical signs can be mild, risk groups, such as pregnant patients with compromised immune systems, can suffer very serious side-effects, leading to congenital birth deformities, blindness, dementia and even death.

The true scale of the hidden problem has shocked experts who believe not enough is being done to warn the public of the known risks posed by toxoplasma, which they judge to be one of the worst food-borne illnesses because of the severity of its effects.

Some experts call in The Independent today for the condition to be made a notifiable disease in England and Wales – meaning that medical staff must be put on alert – bringing the two countries on a par with Scotland, where infections must be reported on a national database. Others question whether families with young children should have pet cats, while some say advice on cooking lamb and preparing vegetables should be changed.

In addition to infections caused by direct contact with cats, people can pick up the parasite by eating the meat of infected animals or from raw vegetables that have not been washed properly to rid them of any toxoplasma eggs contaminating the soil.

About 80 per cent of infected people show no obvious symptoms of toxoplasma and are completely unaware that they are harbouring the parasite. However, new estimates suggest that up to 70,000 people a year in the UK develop some kind of symptoms.

Experts are especially concerned about the emerging scientific evidence suggesting that apparently healthy people with toxoplasma may still be affected unwittingly by the parasite, even when they show no obvious clinical symptoms.

A number of small-scale studies suggest that toxoplasma infection may alter people's personality, making them more prone to risk-taking or delayed reaction times. Studies have also linked toxoplasma infection to psychotic disturbances such as self-harm and suicide, and to serious psychiatric illnesses such as schizophrenia.

This week the Food Standards Agency (FSA) will publish a "risk profile" of toxoplasma in the food chain, The Independent has learnt.

The group of experts commissioned to write the report estimates that 350,000 new toxoplasma infections occur each year in the UK, most of them probably from eating contaminated food.

Experts contacted by The Independent have urged the FSA to review its advice to pregnant women and immune-compromised patients and have strongly advised it to change its policy stating that it is safe for people to eat rare lamb.

Sheep are thought to pick up the parasite by eating pasture grass or concentrated feed that is contaminated with cat faeces and preliminary studies indicate that nearly 70 per cent of British sheep have been exposed to the feline parasite.

Although pregnant women and patients with compromised immune systems are warned to avoid pink meat, the FSA's chief scientist, Andrew Wadge, said that it is safe for people to serve lamb rare, even though one study found that two thirds of lamb samples from one Manchester butcher tested positive for toxoplasma.

"People traditionally eat and enjoy lamb cooked rare, the same as beef. That's how people enjoy it and for most people that is perfectly safe," Dr Wadge said.

"Our advice is always on the basis of what we know and the science changes. I'm not going to tell you about the safety of lamb based on another five or ten years of research, I'm going to tell you what I know now, and there is a long history of people eating rare lamb without any adverse consequences," he said.

However, other experts warned that much of the lamb sold in British supermarkets is likely to be contaminated with toxoplasma cysts in the muscle tissue, which can survive cooking when meat is served pink.

"I would steer very well clear of rare lamb. I would certainly not recommend eating rare lamb," said Barbara Lund, a microbiologist at the Institute of Food Research in Norwich.

"Regarding the comment that it is safe to serve whole cuts of beef and lamb rare as long as they have been properly cooked on the outside, it is not clear to me that we can be confident of this advice for sheep meat," Dr Lund said.

Fuller Torrey, an expert on schizophrenia and toxoplasma at America's Stanley Medical Research Institute in Maryland, said that the seriousness of the potential risks posed by the parasite to the general public means that all meat should be cooked thoroughly to kill parasitic cysts lying dormant within muscle tissue.

"Eating any meat that is rare or undercooked is not safe. I would not advise anyone to eat undercooked meat given what we know and don't know about this organism," Dr Torrey said.

Richard Holliman, a consultant medical microbiologist at St George's Hospital in London, who chaired the FSA's working group on toxoplasma, said that based on existing scientific evidence it is not yet justified to change the official advice on the safety of eating rare lamb for the general public.

"Certainly for pregnant women, the advice is to eat meat that has been thoroughly cooked through, but it's difficult to advise the wider population because you have to balance the risk against people's personal tastes," Dr Holliman said

"Some people enjoy food if the meat it not well done. To them it would be a disbenefit to cook meat until it is well done," he said.

Dr Holliman pointed out that vegetarians also suffer from high levels of toxoplasma infection which indicates that meat is not the only source of food-borne contamination.

"Toxoplasma infects a lot of people but only has an impact in terms of lifestyle on a small proportion of them," Dr Holliman said.

"Toxoplasma is more important, or as important as salmonella and campylobacter, which affect a lot of people. Toxoplasma affects a few people but when it does affect them it can be devastating. A child born with congenital toxoplasma is damaged for life," he said.