The Home Office has been given access to the NHS records of more than 6,900 people since 2010 as part of its efforts to track down illegal immigrants, prompting concerns from patients' and migrants' rights groups.

Medical records are protected by data protection laws but new statistics show the Home Office has made use of a little-noticed exemption in the rules to access patients' non-clinical records, without any need for a court order.

The exemption allows officials to see where people have made use of the health service and when, but not the details of the clinical conditions or medical attention they received.

Police forces and the National Crime Agency have also accessed these records, the figures show, in pursuit of perpetrators of serious crimes such as murder and rape.

Patients' groups said the use of NHS records by immigration and law enforcement officials could deter people from seeking treatment for themselves or their families, and so pose a public health risk.

"We are concerned to learn that the Home Office is seeking to track down potential immigration offenders by requesting their medical records," said Katherine Murphy, chief executive of the Patients Association. "Whilst we are aware of the government's need to clamp down on illegal immigrants, we don't believe that it should be done via their medical and social care records.

"There is the very real danger that some patients may put off seeking medical help for fear of their details being leaked. Furthermore, children may suffer if parents are reluctant to take them for treatment when they are ill. The health and welfare of patients must always be the priority and we must not introduce policies which could undermine the core principles of our health service."

The Home Office requests first emerged in a 91-page independent report on data release policy at the Health and Social Care Information Centre (HSCIC) this month. The PricewaterhouseCoopers report found that between July 2010 and December 2013 the Home Office made more than 12,587 requests for records from the NHS's National Back Office, which keeps basic information on all registered NHS patients.

The vast majority of these requests for information – 12,204 – were accepted by the NBO, and records were found in 6,304 cases. Police and Soca (now the National Crime Agency) made 14,951 requests over the same period. Records on requests prior to July 2010 had been destroyed, the report found.

The records are released under an exemption to the Data Protection Act, which usually protects the confidentiality of NHS records, for the pursuit of serious crime. The NBO passes on basic data on the patient, including the local NHS area the patient last had contact with. The Home Office or police force concerned is then free to pursue more detailed information from local NHS agencies.

A supplemental release of information covering the first three months of 2014 showed the Home Office had since overtaken police forces as the primary requester of health records, making 725 requests for information compared to 635 from police and the NCA.

Ruth Grove-White, the policy director of the Migrants' Rights Network, called the findings "disturbing" and questioned whether the data requests were a reasonable use of strained NHS resources.

"NHS staff are already under pressure and the last thing we need is for them to be forced to act as immigration officers as well," she said. "This further evidence that the Home Office is using the NHS as a vehicle for immigration enforcement is disturbing.

"Some migrants who are entitled to access the NHS are already too intimidated to seek the care they or their children need. In effect, this approach discourages people from going to their GP or hospital when they need to, with wider public health implications."

The Home Office declined to respond to queries about how many of the 6,965 people whose NHS information it had received were further pursued through their local NHS bodies or GP surgeries, but it defended its use of NHS records.

"We work closely with a wide range of partners across government and the private sector to ensure those who are in the UK illegally are removed," said a spokesman. "Any requests for information are proportionate and in line with data-sharing legislation."

The National Crime Agency stressed that its use of NHS records was strictly restricted to serious crimes.

"The NCA uses National Back Office data as one of a number of techniques and intelligence sources to trace known fugitives," said a spokesman. "The use of these inquiries is proportionate and can only be made for serious offences, for example murder and serious assault, and we make them only after all other checks including open source have also been pursued."

The HSCIC stressed that the data requests covered only non-clinical data, and said obtaining medical records would require a court order. It added that it had never received an order seeking medical records.