An official inquiry will find that the mayor of London wrongly ousted Cressida Dick as commissioner of the Metropolitan police, the Guardian has learned.

The findings come from a special commission conducted by Sir Tom Winsor, the former chief inspector of constabulary, after Dick’s decision to resign in February.

A draft of the government-ordered report finds that Sadiq Khan did not follow due process and that Dick was unfairly treated, branding the mayor’s actions and decision-making as “irrational” and “unreasonable”.

A furious Khan is consulting lawyers, and believes the report to be biased and factually flawed. The report also says the mayor’s treatment of Dick was “oppressive”, and that she was placed under unfair pressure.

In a protest letter to the home secretary, Priti Patel, Khan said Winsor had publicly championed Dick, whose commissionership ended in a series of scandals engulfing the Met.

Khan wrote: “While Sir Tom has inexplicably refused to provide me with a final version of his report to you, his draft findings … were clearly biased, and in many instances outside his terms of reference and unsupported by the evidence.”

The letter accused Winsor of treating the claims of Dick and her allies about her treatment as truth: “It is clear from the draft that was provided and subsequent exchanges that he intends to present as fact accounts from the previous commissioner and her team, while ignoring or downplaying the extensive contradictory evidence provided.”

Winsor is expected to argue that he did a thorough job and was fair and balanced, with his conclusion based on the facts. His report went to the home secretary earlier this week and may be published next week.

A home office spokesperson said: “The decision on the publication of the report will be made in due course.”

Winsor was chosen to conduct the inquiry by Patel after standing down in March as the chief inspector of constabulary after 10 years.

Khan made his decision after losing confidence in Dick’s understanding of the depth of the Met’s problems and her ability to stop the force losing public support during her five years in office.

Two months after Dick left, the Met suffered the humiliation of being placed into special measures for the first time since its foundation in 1829. The decision was made by Winsor’s successor as chief inspector of constabulary.

As well as being mayor of London, Khan is the police and crime commissioner for the capital, and is jointly responsible with the home secretary for policing in London. After a public clash with him, Dick chose to resign rather than attend a meeting with Khan, after he made clear that her plans for reform were inadequate.

In his letter to Patel, Khan said of Winsor: “He appears to have omitted many facts relevant to [Dick’s] departure as commissioner, including any of the abhorrent scandals that happened on her watch – including the photographing [by force members] of the murdered sisters Bibaa Henry and Nicole Smallman and then sharing these with other serving officers, and the murder of Sarah Everard by a serving officer.

“Sir Tom Winsor has failed to recognise almost any of the litany of scandals that occurred in the last two years, many of which you and I discussed at the time, which led to my loss of confidence in the commissioner.”

Multiple sources close to Patel, in the months before Dick’s departure, told the Guardian of their grave concern over the Met under Dick. Crucially, neither the Home Office nor Boris Johnson’s Downing Street attempted to publicly save her when she fell out with the mayor.

The procedure for removing a commissioner is set out in legislation, which requires the mayor to set out the reasons for their loss of confidence in writing. Khan did not, arguing that the requirement did not apply because Dick chose to resign rather than attend a meeting with him.

A spokesperson for the mayor said: “The mayor lost confidence in the commissioner after a series of scandals saw trust in the Met police plummet to record lows. We will not provide a running commentary on leaked draft reports.”

The report from Winsor represents a victory for Dick and her allies. Her deputy, Sir Stephen House, had demanded the inquiry, claiming due process had not been followed.

Dick left with a severance package of £166,000, negotiated with City Hall. Her replacement, Sir Mark Rowley, is due to start as commissioner on 12 September.

Winsor declined to comment.