Three more premises have been raided by officials investigating the horse meat scandal.
Two plants in Tottenham, north London, and one in Hull, Yorkshire, were visited by the Food Standards Agency (FSA) on Thursday night.
On Friday it announced: "Computers and documentary evidence have been removed from these premises, as well as meat samples that have been taken for testing.
One of the plants raided in north London, Dinos & Sons Continental Foods, confirmed it was "co-operating with local trading standards officers and the FSA".
The plant, which is based in the Millmead Industrial Estate in Tottenham, issued a statement saying: "Dinos & Sons has been asked to clarify its position in respect of the transportation and storage of frozen beef that was imported by, and belonged to, a third party that the FSA is investigating.
"Dinos & Sons did not keep this frozen beef in the same premises as its own products and there was therefore no possibility of any cross-contamination.
"At no time has Dinos & Sons produced or manufactured anything that is under investigation or is the subject of any possible contamination or mislabelling.
"There is no suggestion whatsoever that Dinos & Sons manufacturing processes have been compromised in any way. Tests undertaken by independent laboratories on Dinos & Sons products have proved negative to date for any contaminants, including horse meat."
On Friday the FSA released eagerly awaited test results for possible horse meat contamination.
The watchdog said 2,501 tests were conducted on beef products, with 29 results positive for undeclared horse meat at or above 1%.
These 29 results related to seven different products, which have already been reported and withdrawn from sale.
The products linked to the positive results were confirmed as Aldi's special frozen beef lasagne and special frozen spaghetti bolognese, the Co-op's frozen quarter pounder burgers, Findus beef lasagne, Rangeland's catering burger products, and Tesco value frozen burgers and value spaghetti bolognese.
As the results were confirmed, pub and hotel group Whitbread became the latest company to admit horse DNA had been found in its food, saying their meat lasagnes and beefburgers had been affected.
The firm, which owns Premier Inn, Beefeater Grill and Brewers Fayre, said the products had been removed from their menus and will not be replaced until further testing has been carried out.
Horse meat was discovered in school dinners for the first time since the scandal began, it was also revealed on Friday.
Cottage pie testing positive for horse DNA was sent to 47 Lancashire schools before being withdrawn.
The Local Authority Caterers Association (LACA), which works with school caterers, said it was currently attempting to assess the situation, adding that the school meals case in Lancashire is the only one it had heard about.
"It appears to be a very small, isolated situation," a spokeswoman said.
"We are trying to assess that. There's a lot of testing been going on in the last week."
Three men who were arrested yesterday on suspicion of offences under the Fraud Act remained in police custody tonight.
Dafydd Raw-Rees, 64, the owner of Farmbox Meats near Aberystwyth, was arrested along with a 42-year-old man.
A 63-year-old man was also arrested on suspicion of the same offence at Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.
Both plants were inspected on Tuesday by the FSA.
The release of today's test results could begin to help restore confidence to shoppers.
But it would likely be "impossible" to ever know the full extent of horse meat mis-labelling, said Steve Wearne, FSA director in Wales.