Jeremy Hunt has ordered health chiefs not to block staff from speaking out as he launched a review into the treatment of whistleblowers.

Sir Robert Francis, QC, who led the public inquiry into the Mid Staffordshire scandal, has already begun taking evidence from whistleblowers who claim that NHS trusts and unions are silencing those who raise the alarm over poor care.

In a letter seen by The Times, the health secretary told the heads of English NHS organisations to set aside any gagging clauses.

“We believe that it is crucial that we hear the voices of those who say they have suffered detriment in the past and learn lessons for the future,” he wrote.

“My expectation would be that the existence of any confidentiality clauses, for example in employment contracts or settlement agreements, would not be used as a justification to prevent NHS staff from contributing evidence and their experiences to the review.”

A source close to Mr Hunt said: “The appalling events at Mid Staffs showed what can happen when staff are not able to raise concerns about standards of patient care, and though we have made progress since, Jeremy wants Sir Robert to have access to all the information he requires to make rigorous recommendations.”

David Drew, a former NHS paediatrician and one of the six campaigners whose appeal for justice helped to prompt the review, said many would-be whistleblowers would still be reluctant to come forward.

“It is surprising that the secretary of state has to remind trust chairs that staff must not be prevented from making statements in the public interest because they have signed a compromise agreement,” he said. “What could be more public interest than the Francis review?”