London (CNN) -- A Europe-wide scandal over horse meat in products labeled beef spread still further Friday, as UK authorities revealed the results of DNA testing on beef products and raided the premises of three more UK food firms.
Of 2,501 tests carried out on beef products across the industry by noon Friday, 2,472 found no horse meat content above 1%, the UK Food Standards Agency said.
The 29 positive tests involved seven products sold by five suppliers, according to the Food Standards Agency.
Another 962 tests are still under way, the agency said at a news conference.
Fifteen of the positive tests were for the lasagna products sold by frozen food giant Findus that first triggered the horse meat alert last week.
The others concerned beef products sold by supermarket chains Tesco, Aldi and The Co-operative, and burgers made by catering supplier Rangeland.
Tesco, Asda and Aldi all issued statements saying they are boosting testing on meat products to protect customers, restore confidence and ensure product quality.
Jim Smith, group technical director for Tesco, said the company will "no longer work with the suppliers who fell below our very high standards."
The Food Standards Agency declined to give details of the names or location of the three food premises raided Friday.
Investigations are ongoing, but authorities cannot rule out the possibility of arrests, it said.
The latest raids come a day after UK authorities arrested three workers at two meat plants, Farmbox Meats near Aberystywth and Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.
Inspectors toured the plants Tuesday and suspended their permits to operate Wednesday, the agency said.
Meanwhile, authorities in northern England confirmed Friday that a dish had been pulled from 47 school kitchens after tests revealed horse DNA.
The ready-made cottage pie, or shepherd's pie, came from an external supplier, the Lancashire County Council said.
"This does not appear to be a food safety issue but I've no doubt parents will agree we need to take a very firm line with suppliers," councilor Susie Charles said in a prepared statement.
Authorities across Europe have been scrambling to get a grip on the crisis over rogue horse meat in beef products.
Fears of mislabeled meat also spread to the sky, where companies that provide in-flight catering in Europe initiated reviews of their suppliers.
LSG Sky Chefs said it has contacted all its meat suppliers in Europe and has asked for written confirmation that their products do not contain horse meat. Another major caterer, Gate Gourmet, is doing the same with its suppliers.
The European Union intends to begin testing meat across all 27 member states, it confirmed Friday.
It called for testing 10 to 150 samples per country and at least five tests per country for the presence of the drug phenylbutazone, also known as bute, which is approved for horses but is not allowed to enter the food chain because it can be harmful to humans.
Over the past week, unauthorized horse meat has been discovered in a variety of products labeled as beef that were sold in supermarkets in countries including Britain, France, Sweden, Switzerland, Germany and Ireland.
In the UK, catering giant Compass Group and Whitbread, which owns hotels, coffee shops and restaurants, were the latest to say Friday that they had found horse DNA in certain beef products.
Whitbread said it was removing a meat lasagna and a beef burger from its menus and would work with the Food Standards Agency to implement a robust future testing regime.
"We are shocked and disappointed at this failure of the processed meat supply chain," it said in a written statement.
Compass Group said an affected burger from Rangeland Foods had been provided to some sites in Ireland and Northern Ireland where it holds the catering contract. It promised DNA testing across processed meat products in future.
NorgesGruppen in Norway also confirmed to CNN on Friday that horse meat had been found in frozen lasagna dishes in its stores.
"The analysis tells us that the lasagnas contained 60% or more horse meat," a spokeswoman said. "We have withdrawn up to 8,000 products last week. We are in talks with the factory, the French company Comigel."
Comigel was one of two French firms whose role in the scandal was highlighted at a news conference held by French authorities Thursday.
The other firm, Spanghero, should have known that the meat it labeled as beef was actually horse, French Consumer Affairs Minister Benoit Hamon said.
Spanghero was the first company to label the meat as beef, the minister said, adding that 750 tons of horse meat were involved over a period of at least six months.
Spanghero should have identified the meat as horse from its Romanian customs code, as well as its appearance, smell and price, he said.
Comigel also should have noticed anomalies in labeling of the meat it received, Hamon said.
A Spanghero representative told CNN the company had acted in good faith. "The company has never ordered horse meat and we never knowingly sold horse meat," the representative said.
The affair has been passed to the Paris prosecutor to be investigated as fraud, Hamon said. The offense is punishable by up to two years in prison and fines of up to €187,500 for the companies involved.
Hamon said there is no reason to doubt that the Romanian abattoir that supplied the horse meat was acting in good faith.
In another twist, UK inspectors said Thursday that horse carcasses contaminated with the equine painkiller bute may have entered the food chain in France. UK and French authorities are working to trace the horse meat, the Food Standards Agency said.
The meat industry was first thrust into the spotlight last month when Irish investigators found horse and pig DNA in hamburger products. The discovery of pig DNA in beef products is of particular concern to Jews and Muslims, whose dietary laws forbid the consumption of pork products. Jewish dietary laws also ban the eating of horse meat.
CNN's Claudia Rebaza, Kendra Wates and Susannah Palk contributed to this report.