The Food Standards Agency has vowed the investigation into the horsemeat scandal will be "relentless" and promised the agency would get to the bottom of the matter after police raided two British meat companies.

The FSA director of operations, Andrew Rhodes, said investigations would continue until "there was nothing left to find".

Pressure on the FSA is mounting after police raided two British meat companies on Tuesday in their first action – jointly with food standards officials – in the food fraud and the horsemeat scandal.

The environment secretary, Owen Paterson, is travelling to Brussels on Wednesday for an emergency a meeting of European countries caught up in the scandal.

Officers entered Peter Boddy Licensed Slaughterhouse in Todmorden, West Yorkshire, and Farmbox Meats Ltd near Aberystwyth in Wales on Tuesday as they investigated the circumstances in which horsemeat was sold as beef "for kebabs and burgers".

But Rhodes said there was, as yet, no evidence to suggest a wider problem in the UK.

"What we have been doing at the FSA is investigating a really broad range of different things and we have actually exonerated quite a lot of businesses so far in our investigations and I am sure that will continue to be the case," he said during a BBC Breakfast interview.

"What we are doing is focusing on the areas we think are the highest risk so we have identified documentary evidence that has led us to take the action that we have. We don`t have evidence that this is a widespread problem in the UK."

Rhodes said five slaughterhouses in the UK processed horses on a regular basis, adding that suspicions about one of them had led to the raid near Aberystwyth. "We will continue with that investigation and we will keep pursuing this until there is nothing left to find," he said.

He added that the FSA had seized all meat as well as paperwork and was working with police in Dyfed Powys and West Yorkshire.

Slaughterhouse owner Peter Boddy said he would co-operate with FSA officers and claimed they had not "raided" his Todmorden premises.

He told ITV: "It was not a raid – they are welcome to visit whenever they want. They just wanted to see my records, which I will be showing them."

Dafydd Raw Rees, of Farmbox Meats, told the BBC that the firm was licensed to deal with horses and it had been cutting horsemeat, from the Irish Republic, for the last three weeks.

"As far as I am concerned, I know nothing about the plant in West Yorkshire. I have never knowingly processed horsemeat until three weeks ago," he said.

"There is nothing we have done here which is not totally permissible."

The FSA has ordered food businesses to carry out tests on all processed beef products and the first results are expected on Friday, although full results could take much longer.

"We`re progressing very well through our investigations but they`re not complete yet. So I`m not going to speculate on what else we might find. But of course, we don`t really expect to find anything because if people are behaving according to the law and doing what they should be doing then there should be nothing to find," Rhodes said .

"But that doesn`t mean that we are in any way complacent. We`ve been very relentless in this. We`ll continue following it through until there is nothing left to find."

On BBC Radio 4`s Today programme, Rhodes denied claims that a surveillance programme had been effectively stopped previously under pressure from the supermarkets. "We still actively sample lots and lots of products," he said.

Paterson held a second meeting with the FSA, food retailers and suppliers on Tuesday to discuss a new regime of random testing of foods. In Brussels he will discuss the growing Europe-wide crisis with the health and consumer commissioner, Tonio Borg, and his opposite numbers from France, Ireland and Romania.

The scandal spread on Tuesday when upmarket retailer Waitrose withdrew its Essential British Frozen Beef Meatballs after pork was detected in two batches.

They had been produced at an ABP Freshlink site in Scotland last summer. The plant, whose closure was announced last year, was not implicated in earlier horse or pork contamination scares at the Irish food group`s Silvercrest and Dalepak sites, in County Monaghan, Ireland, and North Yorkshire respectively.

Waitrose said in a statement: "Several tests have been done on this product and, even though the results have been contradictory, we have taken the precautionary action of removing the frozen meatballs from sale and putting up customer information notices in all our branches."

The meatballs are safe to eat, but pork is not listed as an ingredient and should not be part of the recipe. Essential Waitrose Meatballs and 16 British Beef Meatballs 480g have a best-before date of the end of June 2013 and the end of August 2013.

Several supermarkets have been forced to withdraw burgers, ready-made lasagne and bolognese, but Tuesday`s raids were the first to involve kebab meat.

"I would be appalled if these allegations are proven," said Alun Davies, the Welsh government minister for agriculture. His department was working closely with the Department for Environment, Food and Rural Affairs (Defra) and the FSA "to ensure this matter is dealt with swiftly and decisively", he added.

Mary Creagh MP, the shadow environment secretary, said: "I welcome the action taken tonight by the FSA and the police. I`m glad the FSA has investigated the concerns about horsemeat entering the food chain I first raised with ministers three weeks ago.