The environment secretary is due to meet the Food Standards Agency, food suppliers and retailers on Saturday to discuss the horsemeat scandal after Aldi became the latest supermarket to confirm its withdrawn beef products contained up to 100% horsemeat.

Owen Paterson said it was unacceptable that consumers were mis-sold products, but that the problems originated overseas.

“We believe that the two particular cases of the frozen burgers from Tesco and the lasagne from Findus are linked to suppliers in Ireland and France respectively. We and the Food Standards Agency are working closely with the authorities in these countries, as well as with Europol, to get to the root of the problem,” he said.

Paterson said he believed the food was safe but urged consumers to return products to the retailers. “The French authorities are saying they are viewing the issue as a case of fraud rather than food safety. Anyone who has these products in their freezer should return them to retailers as a precaution.”.

Findus denied reports that the company first knew there was horsemeat in its products last year.

“Findus want to be absolutely explicit that they were not aware of any issue of contamination with horsemeat last year,” it said in a statement. “They were only made aware of a possible August 2012 date through a letter dated 2 February 2013 from the supplier Comigel. By then Findus was already conducting a full supply chain traceability review and had pro-actively initiated DNA testing.”

The Metropolitan police said in a statement it was not carrying out a criminal investigation. “Although we have met with the FSA we have not started an investigation and will not do so unless it becomes clear there has been any criminality under the jurisdiction of the Metropolitan police service.”

Aldi said it felt “angry and let down” by its French supplier Comigel after tests on Today’s Special frozen beef lasagne and Today’s Special frozen spaghetti bolognese found they contained between 30% and 100% horsemeat.

Comigel, which also produced the contaminated Findus beef lasagnes, has blamed its suppliers. Erick Lehagre said he believed his company was buying French beef from a company called Spanghero but it had since told him it had come from Romania.

A spokesman for Aldi said random tests had shown that the products they had withdrawn contained between 30% and 100% horsemeat.

“This is completely unacceptable and like other affected companies, we feel angry and let down by our supplier. If the label says beef, our customers expect it to be beef. Suppliers are absolutely clear that they are required to meet our stringent specifications and that we do not tolerate any failure to do so,” he said.

The company added that it would test the meals for the veterinary drug phenylbutazone, often referred to as bute, but said it was confident the meals were safe.

Hospitals and education authorities were also checking the food they provide for traces of horsemeat. A spokeswoman for the Local Authority Caterers Association said: “We are as sure as we can be that this is not affecting the school catering area.”

She said there were strict guidelines around food safety and supplying dinners in schools, including transparency and traceability of ingredient provenance, and this was written into contracts.

Food businesses have been told to send test results on all their products to the FSA by Friday but Paterson is expected to tell MPs in a statement on Monday that some suppliers have been complaining to departmental officials that they have come under pressure from supermarket suppliers to cut corners.

As David Cameron indicated that he would have no qualms about eating the sort of processed meat dishes that have been at the heart of the recent scare, authorities insisted there was no evidence that frozen food in general was a risk to human health.

But the FSA advised consumers who had bought affected beef lines from Findus not to eat them. They had not been tested for the presence of phenylbutazone, which is banned in the human food chain. It can cause a serious blood disorder in rare cases.

The Guardian has also established that the FSA has been unable to trace all the horses slaughtered in the UK that tested positive for bute last year. The agency has routinely been testing less than 1% of slaughtered horses for the drug, but found four positives in a sample of 82 carcasses in 2012. It carried out a special additional survey on a further 63 horses last year and found 5% of those contained residues, bringing the total of positives to nine.

The Red Lion abattoir, owned by High Peak Meat Exports, has admitted that two of its slaughtered horses had tested positive for bute “historically” but said this was typical of the industry as a whole and that residue levels were so low as not to be a public health issue. The abattoir is currently under investigation by the FSA for alleged animal welfare abuses, and three of its slaughterers have had their licences to kill horses rescinded. The company said it was the FSA’s responsibility to inspect horses at abattoirs and decide whether they were fit for the human food chain.

The FSA found six of the horses found to contain bute last year had been exported to France, two were still being traced, and one had been allegedly returned to two owners in the north of England for personal consumption. However the family of one of the owners, in Chorley, Lancashire, told officials they had never received the carcass nor expected to receive it.

Some companies have told the Guardian they began testing their own products soon after the first cases were reported in Ireland in mid-January. Full details of the testing requirements will be sent to the industry on Monday, although the agency says companies already have enough information to get on with the job and return results by next Friday.

The agency said evidence of the significant amounts of horsemeat in burgers and lasagne pointed “to either gross negligence or deliberate contamination in the food chain”.

It said two particular cases of horse DNA in frozen burgers from Tesco and the lasagne from Findus were linked to suppliers in Ireland and France respectively. “We are working closely with the authorities in these countries to get to the root of the problem. Our priority remains to protect UK consumers.”

Tesco – which withdrew burger lines after one of its products made at an Irish plant had 29% equine DNA and withdrew lasagne made by Comigel – said it had already begun testing other beef lines at independent laboratories.

Cow and Gate, one of the UK’s major baby food companies, began testing its 14 lines containing beef in the second half of last month. The results were due soon, it said. The company, part of the French-based multinational Danone, has no production plants in Britain but has factories in France and Spain. It insists it can trace meat back to a specific cow. Heinz said it did not source from Comigel and would be responding to the request for testing.

“We only source beef for our baby food recipes as whole muscle meat. We are continuing to keep the issue under close review with our suppliers as more information becomes available about the incident and root cause.”

Baxters and Bird’s Eye were among other companies who said they had begun their own tests. Both said none of their products came from any suppliers so far implicated. The Food and Drink Federation, which represents the interests of the UK food industry, emphasised the “small number” of products where significant levels of horsemeat had been detected so far and said it was “unlikely” the national testing programme would reveal negligence or fraud by other suppliers.

Meanwhile Findus said it knew there was a potential problem with its lasagnes two days before the products were withdrawn. It was looking into claims by the Labour MP Tom Watson that meat used by Comigel may have been suspect since August last year.

Labour has claimed the loss of 700 trading standards officers in three years has made this type of consumer fraud more widespread.

It also points to FSA’s Meat Hygiene Service suffering cuts of £12m in the four years to 2014, with the result that the amount of food checked in laboratories has gone down by as much as 30%.