The private health company Care UK is being investigated by the health service after the x-ray records of 6,000 patients were not processed at an "urgent care centre" the company runs in London.

GPs were informed of the incident last month when NHS managers in north-west London wrote to doctors to tell them that patients at the Central Middlesex hospital "had been sent for x-ray and the provider cannot confirm that the radiology reports have been reviewed for missed pathology". Care UK were awarded the right last year to manage the centre, which sees 50,000 patients at the hospital.

NHS Brent found in March that out of a sample of 300 patient x-rays, 120 required further action from a doctor. The NHS then asked Care UK to check all 6,000 x-rays. The letter from NHS Brent told GPs that "the issue has been logged as a serious incident and a full investigation is under way".

The company – one of the biggest to which the NHS outsources work – said on Tuesday that after examining 1,400 records only 60 patients needed to be called about their x-rays.

Care UK's national medical director, Richard Clapp, said: "I would like to stress that, in all the cases reviewed, no further clinical care has been required.

"The second-stage specialist reviews of the x-ray reports have generally been consistent with the original reports and the level of anomalies is lower than national norms. To date, we have not discovered any patient who has suffered harm as a result of this issue."

An NHS Brent spokesperson said it was working with Care UK to deal with the incident, adding: "NHS Brent took immediate action to launch a full investigation when it was made aware of this issue.

"All patients affected are being contacted directly by Care UK and early findings from this review show that only a very small number need be recalled for further assessment.

"It would be inappropriate to comment further until our investigation is complete."

The urgent care centre is a 24-hour, 365-day service set up to treat minor injuries. The centres are seen as a vital part of NHS plans to replace expensive accident and emergency departments.

Such centres are likely to become a feature of the NHS landscape. North-west London has eight A&E units. But r of a £1bn overspend of £1bn by 2015. Reports suggest that A&Es at West Middlesex, Charing Cross, Chelsea & Westminster, Ealing and Central Middlesex could be axed or scaled down.

Roy Lilley, an NHS commentator who broke the Care UK story on Twitter, said: "At a time when all eyes are on the private sector in the NHS, this does not bode well for the future."

Andy Burnham, the shadow health secretary, said there were "real risks to patient safety and continuity of care in a more fragmented health service, as this worrying case illustrates". He called on the health secretary to "put in place appropriate safeguards".

Care UK is well known in the NHS, running specialist hospitals, independent sector treatment centres, diagnostic and treatment centres, GP practices, urgent care services, walk-in centres and health services in 16 prisons. Although a small part of its £200m healthcare business, the urgent care centre was a symbolic step for the company – running NHS hospital services in an NHS setting.