Eight horses slaughtered for food in the UK have tested positive for the veterinary painkiller phenylbutazone, known as bute, new tests from the Food Standards Agency (FSA) revealed on Thursday.

The minister for food and agriculture, David Heath, told the Commons that 206 carcasses had been tested. Six of the carcasses that tested positive may have entered the food chain in France in the last few weeks, according to the FSA, and efforts were being made to recall them. Heath said the Findus lasagne found to contain horsemeat had tested negative for bute. The FSA confirmed that all tests on the food products analysed so far, including Tesco burgers, were negative.

Heath said: "It is unacceptable that bute at any level has been found in horsemeat. We are investigating and anyone found to have broken the law will be dealt with."

The shadow environment secretary, Mary Creagh, told Heath she had raised the issue of bute three weeks ago: "Can he explain why, up until four days ago, all horses were being tested for bute but were still being released for human consumption? In the middle of a horsemeat adulteration scandal, that is catastrophic complacency."

Dame Sally Davies, the UK's chief medical officer, said: "The trace levels detected are very unlikely to have harmed any human, child or foetus." She said a person would have to eat more than 500 horsemeat burgers to get a harmful dose and that the doses people may have received if bute had got into food were "less than 1% of any dose that had ever given an adverse effect".

The Guardian has discovered that the results from two carcasses that tested positive for bute in 2012 – out of a total of nine – were not reported to the FSA for up to seven months. In July 2012 the veterinary residues committee (VRC), which advises the government, warned that it had repeatedly expressed concern about bute entering the food chain.

The VRC said bute had "the potential for serious adverse effects in consumers, such as blood dyscrasia [a rare but life-threatening condition]".

No bute is permitted in horsemeat for human consumption, but it was found in 2-5% of samples tested between 2007 and 2011, during which time only 50 tests a year were conducted. The horse passport system, which was meant to prevent bute contamination in the 9,000 or so horses slaughtered for meat in the UK each year, was not working, a member of the VRC told the Guardian. The FSA chief executive, Catherine Brown, said this was because people were "breaking the rules".

The FSA tested all 206 horses slaughtered in the UK for food between 30 January and 7 February and found eight positive results for bute. By comparison, only 145 tests were conducted in 2012. Six of the carcasses that tested positive, all slaughtered by LJ Potter Partners at Stillmans Ltd in Taunton, Somerset, were sent to France and may have entered the human food chain. The other two, killed at High Peak Meat Exports in Nantwich, Cheshire, did not leave the slaughterhouse.

The rate of bute contamination found was 4%, meaning that across a year, 360 contaminated carcasses would be expected. The FSA did not test for any other possible contaminants.

The FSA said that, from now on, horse carcasses would not be released from abattoirs until they had received a negative test. Brown said this was now possible because the time taken to get test results had decreased from 14 days to 48 hours.

The new programme of testing all horses for bute at the three abattoirs in England and one in Northern Ireland licensed to slaughter horses for food has cost the FSA £415,000 in three months, just one indication of a mounting bill facing the government and manufacturers because of the scandal. The FSA has also introduced unannounced visits at more than 600 standalone meat cutting plants within England, Scotland and Wales over the next three months.

In the case of the two carcasses that tested positive for bute in 2012 and were not reported to the FSA for several months, the horsemeat is thought to have left abattoirs in May and October but the positive results did not arrive at the FSA until 10 days ago. Only then did officials realise the consignments had gone months before to companies in the Netherlands and France. Safety alerts were then issued to authorities in the two countries.

The FSA would not name the companies in Europe, but the contaminated meat that went to the Netherlands came from the same two abattoirs that returned positive results in the new FSA tests released on Thursday. One carcass tested positive at High Peak Meat Exports in May – one of three positive tests there in 2012. The other carcass was tested in Taunton in October, where the firm LJ Potter rents the Stillmans abattoir for a day a week. It had a total of six positive tests in a year.

Under the system then in force, horsemeat was allowed to leave slaughterhouses while the tests were analysed. The results took about three weeks to arrive.

Test results were analysed by a part of Defra called Fera, the Food and Environment Research Agency, and were then meant to be sent immediately to another part of Defra, the Veterinary Medicines Directorate (VMD), and the FSA. The VMD told the Guardian: "The VMD is confident that the results of these bute tests it commissioned were submitted to the Food Standards Agency as soon as they were confirmed by Fera."

The FSA said it "did not receive these two results". It added that the two agencies would put in place procedures to ensure that, where necessary, "all required follow-up actions are implemented". The new FSA results came on the day that a committee of MPs severely criticised the government's response to the horsemeat scandal, calling it "flat-footed".