Schools in Scotland were told on Friday not to serve frozen beefburgers, while in Wales some supplies to schools in seven local authorities have been withdrawn, as the horsemeat crisis continued to roll on.

The move in Scotland came after a frozen beefburger from a school kitchen in North Lanarkshire tested positive for horse DNA, while in Wales it was discovered that some burgers delivered to schools had been made at the Burger Manufacturing Company (BMC), in Builth Wells, one of the most recent producers to be caught up in the growing scandal.

The developments came as Birds Eye withdrew three beef ready-meals from sale in the UK and Ireland as a precaution after 2% horse DNA was found in chilli con carne made for the Belgian market.

Local authorities in Scotland were advised to "place a hold" on the use of the products following the discovery in a burger at a North Lanarkshire school kitchen.

The measure also applies to council leisure facilities and some social care establishments.

Scotland's rural affairs secretary, Richard Lochhead, said it was "really unacceptable that any school child in Scotland should be eating a burger which has got horse meat in it … Of the thousands of tests, this is the first positive result in our schools but it is one too many. No company should be supplying our schools with food with beef products that contain traces of horse meat."

News of the discovery in North Lanarkshire emerged on Thursday night. The school where the positive test was recorded has not been named.

A local authority spokesman in Lanarkshire said: "The council has notified the Food Standards Agency, as it is required to do, and investigations are continuing.

"Our investigations are focusing on the use of frozen burger supplies during the past three months, the maximum length of time these would be held in storage."

Meanwhile, councils across the country were advised to take frozen beef burgers off the menu as a precaution. The move was confirmed by procurement agency Scotland Excel, which deals with contracts on a national basis.

In Wales, Monmouthshire, Blaenau Gwent, Merthyr Tydfil, Rhondda Cynon Taf, Caerphilly, Powys and Neath Port Talbot councils were affected by the BMC incident. Their school burgers were provided by a food distribution company called Holdsworth Foods , which withdrew BMC burgers as a precaution. Powys council said only five high schools had been affected; Blaenau Gwent said six schools; and Caerphilly said it had withdrawn the product from all primary schools.

James Trevithick, of Holdsworth Foods, said to his knowledge no food it supplied to schools in Wales or England had tested positive so far. It was conducting its own tests.

In England, Lancashire council last week reported horsemeat DNA in cottage pies made by Oak Farm Foods and a number of local authorities have withdrawn beef products from school menus pending testing.

Birds Eye withdrew Traditional Spaghetti Bolognese 340g, Shepherd's Pie 400g and Beef Lasagne 400g lines, because they were made by the same Belgian company Frigilunch, responsible for the chili con carne with equine DNA.

Birds Eye's parent company, Iglo Foods Group, said it had conducted tests on all beef products Europe-wide and only the chili con carne tested positive.

The latest in the apparently non-stop series of revelations of horsemeat in food across Europe came as the Food Standards Agency prepared to publish its latest list of results from horse DNA tests by major retailers and caterers. Last week there were 29 positive results from seven products which were revealed before the full results of 2,501 tests were reported.

Birds Eye apologised to consumers for the "unacceptable" incident in a statement on its website. "The withdrawn products will not be replaced on supermarkets shelves until we have finished our investigations and have complete confidence in this supplier," it said.

The company, like many others shaken by the scandal, has introduced an ongoing DNA testing programme to "help us ensure that we continue to reach the standards that all our consumers expect from our products".