Experts are checking to see if the feed was infected

A spokesman for Defra said the first case was identified more than two months ago and the two others were confirmed at the end of last week.

The animals are aged between 36 and 43 months. The farm is the old county of Dyfed but has not been identified.

Defra said an investigation would look at whether the source of the cluster was infected feed, perhaps from abroad.

A spokesman could not confirm reports that it was the first time three cases born after 1996 had been linked to one farm.

He said: "The incidence of BSE is going down but you still do get cases. It wouldn't be a huge surprise if there was a common cause for this."

Farmers' Union of Wales spokesman Alan Morris said he understood the animals were part of a herd of Friesian dairy cows and as BSE is not transmitted through milk there would be no chance of the disease being passed to humans.

One of the three cows was an offspring of another, he said.

"The most important thing to stress is that there's no chance of any BSE cattle entering the food chain.

'Mad cow' disease

"We have only heard the basics about this case. We don't know which farm it's in," Mr Morris added.

Last month the government said that British beef was still banned in 84 countries following the BSE crisis of the 1990s in which thousands of cows were slaughtered.

Britain is currently rated as a high risk country for the transmission of BSE - or "mad cow disease".

However, Rural Affairs Minister Alun Michael said that BSE cases, which peaked at more than 36,000 in 1992, last year fell to 309.

As a result of the fall in the number of BSE cases, the government announced in December 2004 that the automatic ban on cattle over 30 months old being sold as meat was to be phased out.

It was to be replaced by a system of robust testing of cattle for BSE.

The deaths of about 140 people in the UK have been linked to variant CJD, a neurological disorder associated with eating BSE-infected meat.