Hackers have accessed the personal data of almost 500,000 women who participated in mass screenings for cervical cancer, research agency Bevolkingsonderzoek Nederland has said.

The data breach, which took place at the test processing lab Clinical Diagnostics NMDL in Rijswijk in the first week of July, included the names and addresses of some 485,000 women as well as health professionals, and information concerning referrals.

The data, which include the results of self tests and tests at family doctors’ surgeries, probably goes back several years, a spokesman for Bevolkingsonderzoek Nederland told broadcaster NOS.

“Further investigation will have to show what exactly happened and which data were stolen,” he said.

Letters have been sent to the affected women with more information about the hack. They have been warned to be extra vigilant about possible abuse of the data, including targeted phishing emails.

Bevolkingsonderzoek Nederland has suspended cooperation with the clinic to prevent further leaks. According to the organisation, the hackers could access the information but not change it.

Clinical Diagnostics NMDL declined to comment.