Kebab meat sold by Norwegian group Kuraas to restaurants contained between five and 30 percent pork even though it was marked as halal, the agency found.

"We will file a complaint against the producer," Catherine Signe Svinland, an adviser at the food safety watchdog, told AFP.

"In a halal product, there should be no pork at all and when we find such quantities … we don't believe it's an accident but it is in fact fraud," she said.

The group denied it had intended to mislead customers.

"We buy huge quantities of halal meat and we can show invoices corresponding to what we bought and sold," marketing manager Kenneth Kuraas told news agency NTB.

"Pork ending up in these products is simply due to routines not being followed," he added.

Kuraas later explained that a labeling error may have been to blame.

"Our theory is that it happened when the meat was labeled," he said.

Since pork consumption is prohibited under Islam, the Kuraas company sent a letter of apology to the Islamic Council, an umbrella organisation representing Muslims in Norway.

The Islamic halal method of killing an animal requires its throat to be slit and the blood to be drained.

On Thursday halal chicken sausages served to pupils in central London schools and nurseries were revealed to contain traces of pork.

European countries have stepped up food controls in response to the recent scandal which saw millions of frozen ready meals pulled off supermarket shelves after tests showed meat labelled as beef contained large quantities of horsemeat.