A senior MP has called on the Department of Health to ensure that no action is taken by an NHS trust against a hospital manager who broke a gagging order to speak out about concerns over patient safety.

The former chief executive of United Lincolnshire Hospitals Trust, Gary Walker, on Thursday broke his silence to allege that he was forced out of his job in 2010 because he put patient safety ahead of Whitehall targets.

The trust is one of 14 being investigated by health chiefs over high mortality rates in the wake of the public inquiry report into serious failures at Mid Staffordshire NHS Foundation Trust.

Mr Walker has received a lawyer's letter threatening potentially financially crippling legal action for breaching the terms of the package - reported to be worth £500,000 - agreed in settlement of his unfair dismissal claim.

In an email obtained by BBC Radio 4's Today programme, the trust's solicitors DAC Beachcroft told Mr Walker: "Should you breach the term relating to confidentiality, you will immediately repay to the trust on demand all sums paid under this agreement in full and you agree that we may recover the compensation sum from you as a debt, together with our reasonable costs, including reasonable fees in doing so. You hereby indemnify the trust for any losses suffered as a result thereof."

The message made clear that the gagging order covered not only Mr Walker but also his family and witnesses to his unfair dismissal claim, and told him to withdraw any statements he might have made in an interview with Today, broadcast on Thursday.

He was told that if he had already recorded an interview, he should "inform the Today programme that you no longer wish to participate and would wish to withdraw the statements you have made".

The chairman of the House of Commons Health Committee Stephen Dorrell on Thursday night wrote to Health Secretary Jeremy Hunt, urging him to stop the trust from taking any action which would prevent Mr Walker giving evidence to the committee about the reasons for his dismissal.

Former health secretary Mr Dorrell said it was not "legitimate" for gagging clauses to be used to shut down discussion of issues of patient safety. He called on Mr Hunt to follow the recommendation of the Francis Report into the Mid-Staffordshire scandal for a ban on the use of the orders to prevent whistle-blowing.

Mr Dorrell said he accepted that confidentiality agreements were often required as part of agreements on the termination of employment. But he added: "Where there is a legitimate public concern, in particular about patient safety issues, these are not legitimate issues to be covered by confidentiality agreements."