Food suppliers are releasing information about possible contamination of their ready-meals, with most of the results coming in negative.
Experts warn supermarkets have only been able to test 30% of their processed beef products, but so far Tescos, Sainsbury's, Iceland and Morrisons have announced negative results.
However, Lancashire County Council confirmed horse meat was found in cottage pies which were delivered to 47 schools while UK pub group Whitbread said horse meat was found in its beef lasagne and burgers.
The tests come as Downing Street tried to pile pressure on retailers, in a bid to claw its way out of increasingly negative media coverage.
"It isn't acceptable for retailers to remain silent while customers have been misled about the content of the food they have been buying," a Downing Street spokesman said.
The Department for the Environment, Food and Rural Affairs (Defra) have struggled to seem on top of the row since it broke out, with secretary of state Owen Paterson having to be dragged back to London last weekend for emergency meetings after he retreated to his constituency.
In a public letter, 11 major companies, including Tesco and Asda, said they shared consumers "outrage" and were "working around the clock" to resolve the issue.
"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," the letter said.
"We will do whatever it takes to restore public confidence in the food they buy and eat."
It appears horsemeat has found its way into ready meals sold in Tescos, Iceland, Aldi, Lidl, Asda and Waitrose.
Two men were held by police at Farmbox Meats Ltd and another at Peter Boddy Licensed Slaughterhouse this week, although both firms denied wrongdoing.
The FSA announced yesterday that trace of bute, an equine painkiller which is banned from entering the human food chain, had been found in eight horses killed in the UK.