The government cannot be sure there is no safety risk from supermarket beef products that have been found to contain horse DNA, the head of the UK's leading official food control laboratory has told the Guardian.

Responding to the growing scandal over the contamination of burgers with horse and pig meat, which saw the first of the factories at the heart of the row close its production lines on Friday, the former president of the association of public analysts, Dr Duncan Campbell, said: "All we know is it is not a beefburger. What is it? We don't know. Why was it picked up in Ireland and not the UK, and how long has it been going on? Until we know what the source is of the 'horse' or 'something derived from horse' that has been found in the beef products, we cannot be sure there is no food safety risk."

Campbell is the chief public analyst for West Yorkshire and a leading expert on the quality of meat. He will carry out some of the testing as the official investigation into the horsemeat scandal develops.

He said that it was "a reflex" for the UK Food Standards Agency (FSA) to say there was no food safety aspect to scandals of this sort, despite the fact that the law has clearly been broken, which may also mean that it has been broken in other ways.

He questioned whether raw materials could be coming from slaughterhouses that were not approved for processing meat for human consumption, or from unfit horses destined for the knacker's yard but which had instead ended up in the human food chain.

There could also be risks around residues of medicines used for sick animals but not considered safe for the human food chain, he added.

The official investigation into how large numbers of beef products on sale in the UK and Ireland came to contain horse and pig DNA is now focusing on imported ingredients added to cheap burgers.

Industry insiders have told the Guardian they believe that an ingredient called "drind", dehydrated rind or skin, may be at the heart of the scandal. It is commonly used to bulk up cheap meat products.

Additives made from boiled hide or offcuts of carcasses are typically used to bind in added fat and water and increase the protein levels of economy beef products that have a low meat content. These may legally be identified simply as "seasoning" on the label.

How long the adulteration and contamination discovered by the Irish authorities and now being investigated across the UK this week has been going on is not known. Trading standards officers in the UK only began testing samples from the offending factories this week. They have taken samples to test for horse or pig DNA from the Dalepak plant in North Yorkshire owned by ABP, the Irish meat processing giant that supplied Tesco with the affected burgers. Results are expected shortly.

Supermarkets and suppliers have also been frantically commissioning their own tests on meat products in a desperate attempt to find out the ingredients.

Campbell said there had been a rush on materials needed to do the lab analyses that can detect the wrong species in meat, with some supplies running out.

Tesco has admitted that criminality or gross negligence has almost certainly been involved in the adulteration of the economy beefburgers that were found to contain 29% horse DNA by the Food Safety Authority of Ireland (FSAI) in test results published on Tuesday. In this case, sources have told the Guardian, horse chunks are believed to have been part of the raw material used.

Ingredients and raw materials supplied by European processors to ABP are now the focus of efforts to determine how the adulteration took place.

ABP and a further company whose beef produce was found to contain horse DNA, Liffey Foods, supply several other major high street retailers. Beef products sold in Lidl and Iceland contained pork and horse DNA in the Irish tests and have been withdrawn from sale. Asda, Sainsbury's and the Co-op used the same suppliers and have withdrawn lines as a precaution, although the FSAI did not test samples from these retailers because they do not have stores in Ireland.

Burger King also bought burgers from the same ABP Silvercrest factory in County Monaghan as Tesco but said it was confident that its products were made in a different way and were not contaminated.

ABP announced on Friday that it was shutting its Silvercrest factory following a further round of tests by the Irish agriculture department on samples collected this week in which most also contained traces of horse DNA.

The FSAI stood by its assessment that there was no risk to consumer health from the presence of horse and pig DNA in beefburgers. Any bacterial contamination would be killed by cooking, it said. It also revealed that it had retested all the burgers that were positive for horse DNA to check for the presence of phenylbutazone, a commonly used medicine in horses that is not allowed in the food chain, and all of the results were negative.

A Food Standards Agency spokeswoman said: "The FSA is stressing that, on the basis of the evidence, there is no food safety risk to consumers from these products. There is nothing about horse meat that makes it any less safe than other meat products. The meat products were supplied to the retailers by approved establishments. The burgers that contained horse DNA were tested by FSAI for the presence of phenylbutazone, a commonly used medicine in horses that is not allowed in the food chain; all of the results were negative."

The Environment, Food and Rural Affairs select committee announced on Friday that it would be summoning the minister and FSA to answer questions on 30 January about the effectiveness of UK checks on the food industry.