The horse meat adulteration problem on Findus lasagne may have gone on since August 2012, a Labour MP has claimed.

Tom Watson said a letter sent by Findus to UK retailers on Monday (4 February) stated the company was told by its supplier on Saturday that it could not guarantee exactly what was in its beef lasagne. “Further, at least one supplier told them they couldn’t guarantee the conformity of the product they were supplying from as early as August last year,” Watson wrote on his website this morning.

Retailers have confirmed that they were first contacted by Findus regarding the problem on Monday.

The fact Findus knew on Saturday there was a problem with its lasagne raised serious questions about whether the company had done enough to inform the public, Watson said. “For over a week, Findus have known that they could not guarantee the contents of their beef lasagne for over six months,” he said. “There should have been huge public information campaign, refunds should have been advertised. Yet it was only Wednesday that people learnt that horse meat was in the food.”

He branded the company as “negligent”, and said the public needed to be told when Findus executive would have had a “reasonable expectation” that there was a labelling problem with their lasagne.

Watson made his name with his dogged pursuit and questioning of the Murdoch empire during the phone hacking investigations. The West Bromwich MP has shown a keen interest in the Findus revelations since they broke, publishing a list of key questions he thinks the company and its private-equity owners, Lion Capital, should have to answer about their handling of the problem.

Findus UK was not immediately available for comment.