A food poisoning bug that causes nasty bouts of illness in more than 400,000 people every year is present in two-thirds of supermarket chicken, a survey showed yesterday.

Scientists commissioned by the Food Standards Agency (FSA) found the bug campylobacter in 65 per cent of frozen and chilled chicken sold by Tesco, Asda, Sainsbury's, Morrisons and other smaller stores.

Chicken is the biggest source of campylobacter, the most prolific food poisoning bug in the UK, accounting for 30 to 40 per cent of cases.

With correct cleaning and cooking, it poses no threat to human health. But if spread on to work surfaces or underheated it can cause diarrhoea, cramping and abdominal pain, and can even, in rare cases, prove fatal.

Each year there are about 55,000 confirmed cases, but the FSA said the true total was likely to be eight times that level, or around 440,000. Eighty people died after contracting the bug in 2007.

Although the two-thirds finding is in line with a 70 per cent prevalence found in 2001, the FSA said its results showed it would miss its target of halving campylobacter's incidence in chicken to 35 per cent by 2010.

Other countries have been more successful at eliminating the bug, including New Zealand, which has improved cleaning of abattoirs and washes chicken in chlorinated water.

Chlorination cannot be used in the UK because of a European Union ban, despite being described by the FSA yesterday as "essentially harmless".

Scientists tested 3,274 retail samples of fresh chicken across the UK between May 2007 and September 2008. In addition to high levels of campylobacter, salmonella was present in 6 per cent of samples, slightly higher than the 5.7 per cent found in 2001, but low by historic standards.