Labour last night said the Prime Minister needed to take "decisive action" to get a grip on the food scandal.
Shadow environment secretary Mary Creagh said people would be shocked and dismayed that horsemeat had been found in schools and hospitals.
"David Cameron has got to stop hiding behind the retailers and food industry and take decisive action to get a grip on this scandal now," she said.
"He should order the FSA to speed up its testing so that we have a full picture of just how far this has spread in our communities."
The environment secretary, Owen Paterson, said it was "wholly unacceptable" if products marked beef turned out to be horsemeat, which was why it was so important to undertake intensive testing activity to get the full picture.
He said: "Food businesses now have a lot of work to do. They need to move quickly to complete these tests and they need to show their customers they've taken the right steps to make sure this doesn't happen again."
Parenting website Mumsnet said the greatest concern would not be about eating horsemeat, but about lack of regulation and monitoring.
Its co-founder and chief executive, Justine Roberts, said: "We are bemused at why schools and hospitals have been given until April to test the meat they serve while retailers and food companies were ordered to do it in a week.
"Today's news that horsemeat has been found in cottage pie delivered to schools in Lancashire underlines the fact that the low-cost catering firms who often supply schools and hospitals are, if anything, more vulnerable than the big supermarkets."
She urged the FSA to speed up testing of school and hospital meals. Her comments came as Sheffield Council said it had suspended the use of all processed meat in school meals with immediate effect, as a precautionary measure to protect student safety.
In a letter issued by 11 major retailers, caterers and other food companies including Tesco, Asda and Sainsbury's, they said they understood and shared their customers' "anger and outrage".
"The food industry is determined to restore consumer confidence in the food we sell as quickly as possible," it said. "We can't accept a situation where the trust customers place in us is being compromised by fraudulent activity or even, as alleged, an international criminal conspiracy."