The beef proteins are added to make the chicken absorb extra water in a process called "tumbling" so that it can be sold for vast profits. Thai and Brazilian chicken breasts have been doctored by processors in the Netherlands and imported into the UK in this way for at least five years.

Food safety authorities have been aware of the problem since 1997. But they have only recently developed DNA tests sophisticated enough to pinpoint the beef proteins, according to the leading trading standards officer on the case, John Sandford of Hull city council.

The chicken is widely used in pubs, clubs, restaurants and canteens, by manufacturers of processed chicken products, and may have been distributed to schools.

"Thousands of tonnes of this stuff moves around our motorways daily," Mr Sandford said. "My understanding is that some of these products are marketed aggressively as being difficult for the authorities to detect."

The Irish food safety authority found the beef protein in Dutch chicken. They had been tipped off by the UK food standards agency (FSA) that there were concerns about chicken breast fillets imported from the Netherlands.

A report published by the Irish FSA last month found that more than half its samples contained foreign DNA which was not declared on the label. Seven samples contained bovine DNA, seven pork DNA and three both. Most also either failed to declare how much water was in them or had more water than the label claimed. One sample was 43% water. "The main concern is the source of the undeclared bovine material," the report says.

Andrew Reilly, deputy chief executive of the Irish FSA said: "This amounts to blatant fraud. You can't call these products chicken fillets. We have no idea where this bovine material is coming from." Asked whether he thought that meant there was a risk of BSE, he said: "You might say that. I couldn't possibly comment."

The Irish authorities have now impounded the meat, but the UK FSA has not acted to remove the same brands from sale in this country.

Peter Smith, chairman of SEAC, the government's advisory committee which monitors BSE risks, confirmed that the FSA had formally alerted him to the Irish findings. "If you don't know the source of the material, there is clearly a potential for a risk."

The Irish FSA report names brands by the Dutch companies Lelie, Vrieskoop, and de Kippenhof as containing bovine DNA. The Lelie brands also contained pork DNA. All were chicken imported from Thailand and Brazil through the Netherlands where they had been "tumbled" with water and additives containing hydrolysed proteins. These are proteins extracted either chemically or at high temperatures from old animals or parts of animals not normally used for human food such as hide, bones, feathers, skin and ligaments and then injected with water into the chicken.

Lelie's quality manager Ronald Buis acknowledged that the proteins injected in his chicken came from beef. The powder was supplied by Vaessen and Schoemaker he said. "All the animals are checked by the government in Holland, or they were checked by some other government."

The Dutch firm Vaessen and Schoemaker refused to disclose the contents of its protein mixes. Teun Lelie, managing director of Lelie, said V&S was not his supplier and denied that he used beef proteins.

Vrieskoop makes the Duke brand of chicken, one of the most popular among caterers in the UK. Sales director Martin van der Arend told us that he did not know how the bovine DNA had got into his chicken and that his tests in private laboratories had not found it. "We think it is something to do with what the chickens ate in Brazil." He added: "We only use proteins from chicken skin to inject our meat. You need that otherwise the water comes out." He had written guarantees from his protein manufacturer that it contained no beef, he said.

The sales director of De Kippenhof, one of whose brands contained 41% water, declined to comment.

UK wholesalers confirm these brands are still available. One said the Duke brand would be suitable for use in schools.

Dayal Sharma, former president of the Hindu cultural society of Bradford, said he thought Hindus, who do not eat beef, would be very angry at the findings. "It's disgusting and a crime. People will not eat chicken if this is happening."

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