Organised criminal gangs operating internationally are suspected of playing a major role in the horsemeat scandal that has seen supermarket shelves cleared of a series of products and triggered concerns about the contamination of the UK's food chain.

Sources close to the Department for the Environment, Food and Rural Affairs and the Food Standards Agency said it appeared that the contamination of beefburgers, lasagne and other products was the result of fraud that had an "international dimension".

Experts within the horse slaughter industry have told the Observer there is evidence that both Polish and Italian mafia gangs are running multimillion-pound scams to substitute horsemeat for beef during food production. There are claims that vets and other officials working within abattoirs and food production plants are intimidated into signing off meat as beef when it is in fact cheaper alternatives such as pork or horse.

In an attempt to reassure the public that Britain's food chain was not victim to systemic fraud, the environment secretary Owen Paterson on Saturday met representatives from the big four supermarkets, retail bodies and leading food producers to thrash out a plan to increase the amount of DNA testing of food.

"The retailers have committed to conduct more tests and in the interests of public confidence I've asked them to publish them on a quarterly basis," said Paterson. He stressed there was no evidence yet that the scandal had become a public safety issue.

Paterson insisted retailers had to play the leading role in clamping down on the problem. "Ultimate responsibility for the integrity of what is sold on their label has to lie with the retailer."

The last time the government sanctioned testing for horsemeat in animal products was in 2003 when equine DNA was found in salami.

The first results of a new series of tests for equine DNA in what the FSA terms "comminuted beef products" – where solid materials are reduced in size by crushing or grinding – will be published on Friday. "We have to be prepared that there will be more bad results coming through," Paterson said.

He confirmed that the government was open to bringing in the Serious and Organised Crime Agency if, as seems evident, the fraud is on an international scale. He said the Metropolitan police had been asked to investigate the scandal and that the force was liaising with counterparts in other countries. Paterson suggested the scandal was potentially a "worldwide" issue.

"I'm concerned that this is an international criminal conspiracy here and we've really got to get to the bottom of it," he said.

The Labour MP Mary Creagh said she was passing information to police that suggested several British companies were involved in the illegal horsemeat trade. "I hope that this information will enable the police to act speedily to stamp out these criminals who are putting the future of the food industry at risk."

Concerns about the substitution of horsemeat for beef first emerged in mid-January when supermarket chains withdrew several ranges of burgers. Fears of contamination prompted hundreds of European food companies to conduct DNA checks on their products that resulted in the food giant, Findus, discovering that one of its products, a frozen beef lasagne, contained meat that was almost 100% horse.

It has emerged that Findus conducted three tests on its products on 29 January that suggested there was horsemeat contamination. The revelation has raised questions about why it took several days for the products to be pulled from the shelves.

Findus indicated it was ready to sue as the company announced it would on Monday file a complaint against an unidentified party.

In a statement, the firm said: "Findus is taking legal advice about the grounds for pursuing a case against its suppliers, regarding what they believe is their suppliers' failure to meet contractual obligations about product integrity. The early results from Findus UK's internal investigation strongly suggests that the horsemeat contamination in beef lasagne was not accidental."

Supermarket chain Aldi has confirmed that two of its ready meal ranges produced by Comigel, the French supplier also used by Findus, were found to contain between 30% and 100% horsemeat.

Comigel claims it sourced its meat from Romania, which has been subjected to export restrictions due to the prevalence of the viral disease equine infectious anaemia in the country. Spanghero, the French company that supplied the meat for the Findus beef lasagne, announced it will also sue its Romanian suppliers.

The scandal has raised questions about what happens to the 65,000 horses transported around the EU each year for slaughter. The campaign group World Horse Welfare said thousands of animals suffered as a result of making long journeys across national borders. Partly as a result of welfare concerns, the trade in live horses has fallen dramatically. In 2001, 165,000 horses were shipped across Europe.

The decline in the cross-border trade in live horses has seen an increase in the sale of chilled and frozen horsemeat, much of which goes to Italy. Last year Romania significantly increased its export of frozen horsemeat to the Benelux countries.

Attention is now focusing on eastern Europe, a major supplier of horsemeat to France and Italy. Some of the meat that went into Ireland came from suppliers in Poland, which exports around 25,000 horses for slaughter each year. Industry sources also suggested to the Observer that gangs operating in Russia and the Baltic states were playing a role in the fraudulent meat trade.

Other food companies have, as a result of their investigations, found that their supplies have been contaminated. The FSA confirmed that meat held in cold storage in Northern Ireland has been impounded after it was discovered to contain equine DNA. A London-based company, 3663, found pork in some of the halal meat it supplies the prison service.

Questions are now being asked about meat supplied to a range of public sector organisations, including the NHS. "Every NHS and healthcare organisation will have different local circumstances and it would be for those organisations to satisfy themselves that the food they supply meets the needs of their patients," said the Department of Health. "Any investigations into the provenance of those supplies would also be done locally."

British farmers have expressed concerns that the scandal could affect consumer confidence in British beef. "Our members are rightly angry and concerned with the recent developments relating to contaminated processed meat products," said the National Farmers' Union president, Peter Kendall. "The contamination took place post farm-gate which farmers have no control over."