England's higher education regulator has apologised for a string of mistakes in its decision to punish the University of Sussex with a record fine that was later overturned by the high court.

The Office for Students (OfS) tried to impose a £585,000 fine against Sussex last year after claiming the university breached free speech regulations involving its former professor Kathleen Stock. It was forced to back down in April after a judicial review initiated by Sussex found the OfS "had closed its mind" and acted with bias.

In a statement, the regulator said: "The OfS made mistakes in the Sussex case, which we regret. The actions we are announcing today show our continuing commitment to learning the lessons from this case with humility and honesty."

Among the changes promised by the OfS in a "lessons learned" report were "ensuring respectful relationships with higher education providers" and in the future using "alternative forms of intervention where possible to effect change" before taking enforcement action such as fines.

Prof Sasha Roseneil, Sussex's vice-chancellor, said she welcomed the OfS's acknowledgement of its mistakes but was concerned that its plans for reform "fell short" in a number of areas.

Roseneil said: "[The report] reveals deep cultural problems within the OfS in relation to freedom of expression and internal challenge, and it is heartening to see the new leadership setting different expectations for the organisation, expectations that are more appropriate for a public institution, trusted to regulate one of the most important sectors in a democratic society.

"However, the report does not fully reflect the breadth or the seriousness of the high court's findings or all the lessons that could be learned from the case.

"It is also disappointing that the review missed the opportunity to engage with the University of Sussex when their lack of engagement was roundly criticised by the high court."

The high court's findings were backed up by an internal review by KPMG commissioned by the OfS, published in summary on Thursday. KPMG's findings criticised the regulator for being adversarial, allowing gaps in its written records, and lacking clear accountability.

"It appears that insufficient emphasis was placed on maintaining constructive engagement and dialogue with higher education providers, which contributed to relationships becoming increasingly adversarial," according to the review's summary.

KPMG also criticised the OfS's leadership at the time for becoming blinkered: "The investigation team was made up of particularly senior staff, reflecting the case's novelty, complexity and profile.

"However, the concentration of decision making and delivery responsibilities within a relatively small group reduced the opportunity for independent challenge and fresh perspectives."

The OfS's chair, Edward Peck, and chief executives Ruth Hannant and Polly Payne, thanked the University of Sussex's leadership "for the frank discussions and insightful commentary leading to this point".

Hannant and Payne took over as the OfS's chief executives in June and had no prior involvement with the decision to fine Sussex. The OfS investigation began after Stock's resignation from Sussex in 2021, following campus protests for her views on transgender rights and gender identity.