Officials investigating the horse meat mis-labelling scandal will continue examining evidence today after three more plants were raided.

The Food Standards Agency (FSA) visited two premises in Tottenham, London and one in Hull, Yorkshire, on Thursday. Investigators, accompanied by police officers and local authority officials, made seizures at the three sites and removed meat samples for testing.

An FSA statement said: "Computers and documentary evidence have been removed from these premises, as well as meat samples that have been taken for testing." The FSA also said it had passed on evidence to Europol - the European Union's law enforcement agency and the authorities in dozens of countries.

It said: "The FSA has submitted a full file and evidence on this issue to Europol. The agency has continued to provide information to Europol, and this information has now been analysed by both Europol and law enforcement agencies in 35 countries - across Europe and elsewhere."

One of the plants raided in north London, Dinos & Sons Continental Foods, said it was "co-operating with local trading standards officers and the FSA". Bosses at the plant, which is based in the Millmead Industrial Estate in Tottenham, issued a statement saying: "Dinos & Sons Continental Foods Limited confirms that it is co-operating with local Trading Standards Officers and the FSA in respect of all its current investigations.

"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."

The raids came as three men who were arrested on Thursday remained in police custody on suspicion of offences under the Fraud Act.

Dafydd Raw-Rees, 64, the owner of Farmbox Meats near Aberystwyth, and a 42-year-old man were arrested in Wales. 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.

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%. The 29 results related to seven different products, which have already been reported and withdrawn from sale.