An independent process to benchmark the quality of Parliamentary Health Service Ombudsman investigations into NHS complaints should be established, MPs have said.

The report by the Commons health committee also described the treatment of NHS whistleblowers as a “stain on the reputation of the NHS” and said “proven” whistleblowers should receive an apology and “practical redress”. The report comes just weeks before a major review of whistleblowing in the NHS by Sir Robert Francis QC is expected to be published.

In its report on the NHS complaints system and raising concerns, published today, the committee highlighted ongoing concerns by campaigners and patient groups that the ombudsman is failing complainants.

In November the Patients Association announced it would no longer refer patients to the ombudsman - the final arbiter over NHS complaints - because of the poor quality of investigations and the distress this causes families.

Ombudsman Dame Julie Mellor has begun a transformation programme since joining the body in 2012. It now aims to carry out 10 times more investigations than it did in 2012 and is upholding more complaints than ever before.

But the committee report said: “The progress that is being made in increasing the numbers of investigations and in modelling a better complaints system will count for nothing if the public perception of the PHSO is that its investigations take too long, require too much of those who are complaining and do not provide appropriate redress at the end of the process.

“The serious criticisms of the ombudsman from the Patients Association are of grave concern. We recommend that an external audit mechanism be established to benchmark and assure the quality of ombudsman investigations.”

The committee has asked the ombudsman to respond with a timetable for improvements to be made.

On whistleblowing, the committee said: “The treatment of whistleblowers remains a stain on the reputation of the NHS and has led to unwarranted and inexcusable pain for a number of individuals.

“Whilst this committee is clear that professionals have a duty to put patients first and to come forward with their concerns, we recommend that those who have suffered harm as a result of doing so and whose actions are proven to have been vindicated, should be identified and receive an apology and practical redress.”

A spokesman for the PHSO said: “We agree with the health select committee’s recommendation to introduce an external element in to our quality assurance. This is something we are already committed to doing.

“We are also pleased the committee recognises the changes we have made, including giving more people justice by doing thousands of investigations every year.”

NHS Confederation chief executive Rob Webster said: “We support the calls for practical redress for those who have been harmed as a consequence of raising genuine concerns. We should apply the same golden thread of complaints handling to staff who have been failed.”