Tesco has sacked one of its main suppliers over the horsemeat in burgers scandal, the company has said.

Britain's largest supermarket chain said on Wednesday that one of the biggest burger plants in Europe had failed to source, as demanded, all ingredients from the UK and Ireland and had been dropped for "breach of trust". The meat used in Tesco burgers found to contain up to 29% horse DNA had instead come from Poland.

However, ABP Food Group, owners of the Silvercrest plant in Ireland, revealed that Tesco would continue sourcing fresh beef from other ABP companies.

Tesco said the findings from its investigations concurred with those of the Irish government, and that it would now start its own system of comprehensive DNA testing.

Beefburgers on sale in Spain were the latest to come under scrutiny on Wednesday, when tests commissioned by a Spanish consumer organisation found two out of 20 samples were contaminated with horse DNA. Tesco, Iceland, Lidl and Aldi were among supermarkets identified a fortnight ago as having products with equine DNA in them by Irish food safety authorities, although not all the products implicated had been on sale in Britain.

Other supermarkets took their burgers off the shelves as a precaution. Tesco's supplier, Silvercrest in County Monaghan, which normally makes 200m burgers a year for different clients, was one of three food processors identified. The Silvercrest plant has been shut down for deep cleaning, the management has been changed and the Irish government is going to supervise the processor's operations indefinitely.

Investigations at the only UK plant owned by ABP – Dalepak Hambleton in North Yorkshire – are continuing, although it is still in operation.

Tim Smith, Tesco's technical director, said: "We now understand – with as much certainty as possible – what happened. The evidence tells us that our frozen burger supplier, Silvercrest, used meat in our products that did not come from the list of approved suppliers we gave them.

"Nor was the meat from the UK or Ireland, despite our instruction that only beef from the UK and Ireland should be used in our frozen beefburgers. Consequently we have decided not to take products from that supplier in future."

Smith said Tesco was ultimately responsible for the food it sold. "We will not take anything for granted after this incident. It has shown that, in spite of our stringent tests, checks and controls there remained a small possibility that something could go wrong and it did. We want to stop it ever happening again, so we are taking action to reduce that possibility still further.

"To underpin the strong measures already in place, we will now introduce a comprehensive system of DNA testing across our meat products. This will identify any deviation from our high standards."

ABP said it had let customers down and apologised. Paul Finnerty, the group's chief executive, said: "We have learnt important lessons from this incident and we are determined to ensure that this never happens again.

"We have put in place new procedures to audit all our third-party suppliers. We have also established comprehensive DNA testing procedures – we will become an industry leader in this area," said Finnerty, adding the company would not allow the Silvercrest incident "to overshadow what is a great business".

Poland has been named as the source country for the contamination just as the Warsaw government seeks to improve its own population's confidence and loyalty in homegrown beef. A section in the latest edition of the government newsletter Polish Food is devoted to this drive, which comes as beef production there is expected to fall substantially.

In 2012 British imports of fresh and frozen beef from Poland totalled 6,200 tonnes, 3.5 times the amount imported in 2010, although this still represented less than 3% of all imports. Ireland imported just 849 tonnes last year, less than 4% of all imports.

Eblex, the organisation responsible for marketing the English beef and sheep industries, says Poland lacks a specialist beef industry. Instead, young bulls are almost entirely produced from dairy calves.

In a private member's debate in Westminster Hall on Wednesday, the Labour MP for Croydon North, Steve Reed, questioned whether the UK's Food Standards Agency was still fit for purpose.

He said that the government had carved up its responsibilities between the environment and health departments and cut its budget, which, along with "swingeing" cuts to local authority budgets for trading standards, had left food safety and labelling controls fragmented.

Defra minister David Heath said that although the amount of food sampling carried out by local authorities had fallen, tests were more sophisticated. He admitted the number of food safety officers had fallen by 6% between 2010 and 2012.

The minister told MPs that the agency was looking in detail now at how horsemeat from the UK contaminated with the drug phenylbutazone, which is banned in the human food chain, came to be exported to and consumed in other European countries.

Asked if he could reassure the public that contaminated horsemeat had not re-entered the UK food chain from Europe, he said that the department would re-examine how horse passports operated.

The government has been criticised for abolishing the National Equine Database, but Heath said that was a "red herring", since it had never provided information on whether horses were fit for the human food chain.