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Worried shoppers are turning to vegetarian food in the wake of the horse meat scandal.

The UK’s second biggest supermarket, Asda, yesterday confirmed demand for meat products had been hit by the crisis.

And sales of meat-alternative brand Quorn have shot up by 15% and fish by 20%.

Asda chief executive Andy Clarke, said he had been “shocked” by the horse meat revelations.

The firm scrapped thousands of pots of its 500g Beef Bolognese Sauce this month after tests revealed the presence of horse DNA.

It also withdrew Freeza frozen beefburgers.

Mr Clarke said: “The first thing we did was look inside our own organisation to identify if we had anything we should be concerned about.”

He vowed to leave “no stone unturned” to address problems in the supply chain, adding: “There is a significant adulteration of the supply chain.

"This isn’t a UK issue. It is across Europe and maybe the world.”

Around 75% of Asda meat was British, but Mr Clarke said: “One issue is the volumes that are available in the UK to procure.”

His comments came as beef burgers made by a firm in Mid Wales were withdrawn after tests showed evidence of horse meat.

Three samples from the Burger Manufacturing Company, at Llanelwedd, Builth Wells, tested positive in checks done at the request of the Food Standards Agency.

Further work is being carried out to establish how much horse meat the products contained and to test for the veterinary painkiller phenylbutazone, or bute.

John Sparks, director of the company, said the batches of burgers had been recalled and customers told.

He linked the shipments to Farmbox Meats, a firm near Aberystwyth, which was raided last week.

He said: “Our decision to use Farmbox Meats was to help boost local businesses and because of our belief in the quality of their meat.

"It is disappointing that on this occasion the high standards we expect have not been met.”

Labour’s Mary Creagh has asked Environment Secretary Owen Paterson what steps have been taken to ensure food used in the public sector was free from horse meat.

Shadow Environment Secretary Ms Creagh said: “The public must have absolute confidence in the food being served to school children, hospital patients, our armed forces and public servants.”

The FSA is due to publish the results of a second batch of tests on products today.