The Dutch meat trader at the heart of the 50,000-tonne beef recall purchased at least 300 tonnes of horsemeat, and horse DNA has been found in 21% of meat samples taken from his company, Dutch authorities have revealed.

Dutch food safety watchdog NVWA took 168 samples from 1,500 tonnes of meat owned by Willy Selten – a meat wholesaler based in the Dutch city of Oss – and said it found horse in 35 (20.8%) of them. A spokesman for NVWA declined to specify how much horse DNA was found in the samples and was unable to say exactly where the 300 tonnes of horsemeat had come from, although he said some had come from Poland.

According to NVWA, Polish authorities have launched a criminal investigation, which will look at how mislabelled horsemeat was supplied to the Netherlands.

NVWA also revealed meatballs and burgers that tested positive for the veterinary drug phenylbutazone (bute) in Portugal earlier this year had been made with raw material supplied by Selten, and said Selten had received a horse carcase contaminated with bute from the UK in May 2012, which he was unable to trace.

The information was revealed in a written update from NVWA to the Dutch parliament on 24 April. Fifty-thousand tonnes of beef sold by Selten’s two companies – Wiljo Import and Export BV and Vleesgroothandel Willy Selten – were recalled by Dutch authorities at the beginning of April, following a raid on the companies in mid-February.  

The NVWA spokesman said of the 50,000 tonnes, 23,000 tonnes were delivered to companies in the Netherlands, and most of it had probably already been eaten.

At the time of the recall, NVWA said it suspected Selten had supplied beef mixed with horsemeat, but the primary reason for the recall was a lack of traceability on the meat. The NVWA has been criticised in the Netherlands for raising unnecessary alarm with its recall, but defended its actions to parliament saying a recall was necessary because if traceability could not be established, it was impossible to guarantee the meat was safe.

Selten’s businesses had since gone into administration, the NVWA spokesman said.

Although NVWA is responsible for alerting other European Union countries to the recall, it is up to individual member states to follow up on the information and ensure the meat is removed from the market.

On 12 April, the Food Standards Agency said eight UK businesses may have received meat from Selten’s companies, but has not named them to date.