The board of London's Air Ambulance has sacked its chief executive, who was investigating allegations of malpractice at the charity, after only 12 weeks in the job.
David Philpott, who was escorted off the charity's premises by trustees on 17 November, was looking into written and verbal allegations made by "trustees, staff and the wider community", he said.
Philpott said he was planning to take the charity to an employment tribunal on the grounds of wrongful dismissal.
A spokeswoman for London's Air Ambulance, which is also known as Virgin Hems London, confirmed that the trustee board had terminated Philpott's contract "under the terms of his probationary period".
"The board of trustees is aware of various issues which Mr Philpott has raised during the course of his employment and is taking appropriate steps to thoroughly investigate those matters to ensure the trust's continued adherence to best principles of governance," she said.
"In light of the need to observe procedural fairness, no further comment will be made until the conclusion of this investigation."
Former chief executive Andrew Cameron will act as interim head of the organisation until a replacement is appointed.
A spokeswoman for the Charity Commission said concerns had been raised with the regulator and it was assessing what further action might be required.
Philpott, who was previously chief executive of Kent Air Ambulance for six years, said he had been "overwhelmed by the messages of support from so many friends and colleagues" since his dismissal.