Environment secretary Owen Paterson is to hold an emergency meeting with the Food Standards Agency (FSA) and retailers, as ministers warn of a "criminal conspiracy" at the heart of the horse meat scandal.

Frozen food company Findus UK has reiterated its apology after tests found up to 100% horse meat in some of its beef lasagnes, while claims emerged that the meals may have been contaminated since August.

Supermarket chain Aldi confirmed that two of its ready meal ranges produced by Comigel, the French supplier also used by Findus, were found to contain between 30% and 100% horse meat. The scandal became a police matter on Friday as Scotland Yard announced they had met representatives from the FSA.

Prime Minister David Cameron said it was "shocking" and "completely unacceptable", as the FSA ordered firms to carry out tests on all processed beef foods. "People will be very angry to find out they have been eating horse when they thought they were eating beef," he said.

The Trading Standards Institute said the discovery of such high levels of horse meat suggested "deliberate fraudulent activity".

Mr Paterson told BBC News: "We are determined to get to the bottom of what's happened. It's either incompetence but I've got a nasty feeling it's actually a criminal conspiracy. And that's why it's quite right for the FSA to engage the Metropolitan Police who are working with other police forces across the mainland of Europe."

The Environment Secretary will meet heads of food retailers and suppliers at the Department for Environment, Food and Rural Affairs (Defra) in London, a spokesman said.

Findus UK confirmed it carried out a full product recall on Monday, two days before DNA tests found that some of its lasagnes contained up to 100% horse meat.

Labour MP Tom Watson published a letter on his website which he claimed was sent by Findus to retailers on Monday, warning that a France-based supplier had told it there may be problems with raw materials delivered since August 1 last year. Findus was asked to respond to the claim but has yet to do so.

A company spokesman said its priority was providing quality products customers can trust. "But we know that many people have been concerned by the news this week that tests have shown that some of our Findus Beef Lasagne has been found to contain horse meat. We understand those concerns, we are sorry that we have let people down," he said.