Findus's 'beef' lasagne may have been contaminated with horsemeat since last summer, it was claimed today.

Labour MP Tom Watson said he had obtained a letter from the company to retailers warning that a French-based supplier told it on February 2 that raw materials delivered to it since August 1 last year were "likely to be non-conform".

The letter, which Mr Watson said was sent to retailers on Monday, added: "The supplier has asked us to withdraw the raw material batches."

Tesco confirmed today that it was informed of a product withdrawal by Findus on Monday.

Findus was not immediately available for comment.

The allegations came as food companies will be ordered to test their beef products after some Findus beef lasagnes were found to contain up to 100% horse meat, the Food Standards Agency (FSA) said.

It was "highly likely" that criminal activity was to blame for the contamination, the agency added, as consumers were warned not to eat the meals.

The frozen food giant has apologised to customers and said refunds would be offered to anyone who bought the affected lasagne products, which were made by French food supplier Comigel.

Tesco and discount chain Aldi have already withdrawn a range of ready meals produced by Comigel.

Catherine Brown, chief executive of the FSA, said: "This is an appalling situation.

"I have to say that the two cases of gross contamination that we see here indicates that it is highly likely there has been criminal and fraudulent activity involved."

There is no evidence to suggest the horse meat found is a food safety risk, the FSA said.

But the agency confirmed tests have been ordered on the lasagne for the veterinary drug phenylbutazone or "bute", which is banned from entering the food chain.

FSA director of operations Andrew Rhodes stressed that contamination had been found in only two branded products, and that there was no evidence that they posed a risk to public health.

Mr Rhodes said: "What we've identified so far are two areas of gross contamination – the Tesco value burger and the Findus beef lasagne. I can't speculate on what we might find in further testing, but if we do find something then we will be dealing with that very robustly.".