The Parliamentary and Health Service Ombudsman (PHSO) has resigned in the wake of revelations concerning her deputy's role in covering up a sexual harassment case.
Dame Julie Mellor handed in her notice ahead of the publication of a review commissioned after the incident came to light earlier this year.
She admitted that she "made a mistake in handling correspondence" about former deputy PHSO Mick Martin, according to a response to her resignation letter.
Martin, who was also managing director at the NHS complaints watchdog, resigned in April after it emerged he had "assisted" in the cover-up of a sexual harassment case when he was a senior executive at Derbyshire Healthcare NHS Foundation Trust.
An employment tribunal last year heard how Martin assisted the trust chairman, Alan Baines, in covering up his conduct relating to HR director Helen Marks.
The tribunal ruled that Marks had been subject to sexual harassment and bullying.
It concluded Martin and other senior executives "assisted Mr Baines in covering this matter up and preventing any proper investigation into his behaviour".
In April, Martin resigned from his post at the PHSO following a period of paid leave.
The PHSO board appointed Sir Alex Allan to lead an independent review of the organisation's actions in relation to Martin.
The review examined the actions and procedures followed by the PHSO organisation when the findings came to light.
In her resignation letter to Bernard Jenkin, chairman of the Public Administration and Constitutional Affairs Committee, Dame Julie said the PHSO had received the independent review and expected to publish the findings in a few weeks' time.
She said that she was resigning in the "context of the handling of correspondence about our previous managing director".
Jenkin's response states: "You have always been open with me that you made a mistake in handling correspondence about your previous managing director. It is typically conscientious of you to apply the most rigorous standards to yourself."
Dame Julie said she would remain in post until a successor was appointed.
Commenting on the news, Katherine Murphy, chief executive at the Patients Association, said: "The public rightly expects that as an ombudsman, the PHSO should be able to be held up as a shining beacon of outstanding public standards and independent action.
"However, the Patients Association has worked with many families who have been let down by the quality of PHSO investigations.
"In far too many cases the PHSO has failed to investigate, and when it does, the investigations appear to be light touch, conducted at arm's length and biased in favour of the systems being investigated.
"The whole complaints system has overlooked how to treat complainants in a dignified way, whether that's the patient themselves or their families and carers.
"Complaints are a vital part of improving health and care services and should be welcomed.
"Patients must be involved and engaged in the complaints process so they have confidence they will get answers, and stop the mistakes of poor care from happening again.
"Perhaps with the change of leadership at the PHSO this service can now start to carry out its duties with the independence and robustness that the public deserves.
"Patients have been failed by the PHSO for far too long."