PARIS/LONDON (AFP) - Swedish food giant Findus said it will on Monday file a complaint against an unidentified party after horsemeat was found in products supposed to contain only beef.

"We were deceived," the managing director of Findus France, Matthieu Lambeaux, said in a statement. "We will file a complaint against 'X' from Monday."

French frozen food supplier Comigel told the Findus brand that "the contamination" of processed beef products with horsemeat could date back to August 2012, Findus said in a statement Saturday.

"Findus want to be absolutely explicit that they were not aware of any issue of contamination with horsemeat last year," it said in a statement.

"They were only made aware of a possible August 2012 date through a letter dated 2 February 2013 from the supplier Comigel.

"By then Findus was already conducting a full supply chain traceability review and had pro-actively initiated DNA testing." The opposition Labour lawmaker Tom Watson earlier claimed he had been shown by a retailer a copy of a letter that Findus sent to them on Monday.

He published an extract on his website.

"Investigations have led one of our suppliers based in France to inform us in writing on 2nd February 2013 that the raw materials delivered since 1st August 2012 are likely to be non-conform and consequently the labelling on finished products is incorrect," it said.

Britain's Food Standards Agency regulator announced Thursday that 11 of 18 samples of Findus beef lasagne were found to contain between 60 and 100 percent horsemeat.

Asked by AFP to comment on Britain's food minister fearing a criminal conspiracy, a Findus spokesman said: "That's part of our investigations. We need to understand how the meat got into the supply chain. That's something we are working on with Comigel, as the supplier." He did not know how long the investigation would take.

The spokesman said it was "still uncertain... where the meat exactly came from".