British police are investigating the horse meat scandal after tests showed Findus beef lasagnes contained up to 100% of the product, the Food Standards Agency has confirmed.
The FSA said officers in the UK and in Europe were involved in the probe into burgers and lasagne that contained large amounts of horse.
It said the evidence pointed to "either gross negligence or deliberate contamination in the food chain."
Details of the police investigation have not been made public, but more information is expected later on Friday.
Meanwhile, the Department for Education responded to fears that the horse meat may have been served in schools. It said any schools with concerns should "contact their caterer or local authority immediately".
Unions called on the government to confirm that no horse meat had been served to vulnerable users of public services.
Brian Strutton, the GMB union's national secretary for public services said: "Hospitals, schools and council care services provide millions of meals with meat products every week to the young, the elderly and the sick.
"The government must require all public services that feed vulnerable members of society to verify that they have not used horse meat."
His comments came after tests on Findus beef lasagne revealed some contained 100 per cent horse meat.
In a new statement, the company reiterated its apology, amid claims by Labour MP Tom Watson that the contamination could stretch back to last summer.
A Findus spokesman said: "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."
The FSA said: "This is a very serious issue. The evidence we have about the two cases, of the significant amount of horse meat in burgers and lasagne, points to either gross negligence or deliberate contamination in the food chain. This is why we have already involved the police, both here and in Europe."
An international investigation is underway into frozen burgers from Tesco and the Findus lasagne, which are linked to suppliers in Ireland and France respectively.
The Food Standards Agency said there was no evidence of a safety risk. But tests are underway to see whether a banned veterinary drug was contained in the meat, and customers have been advised not to eat it.
Phenylbutazone, or 'bute', is banned from entering the food chain.