Manchester City breached Premier League financial rules by creating “sham” contracts to help inflate revenue and reduce costs by more than £900m over a nine-year period, an independent commission found.

The league announced on Tuesday that City had been found guilty on all charges related to serious breaches of its financial rules between seasons 2009-10 and 2017-18. It said a sanction would be addressed in a further hearing with the commission.

City created a number of commercial deals with sponsors that were part of a scheme to disguise where the funding was coming from, the commission concluded. Companies were required to pay only a portion of the stipulated fees, with the remainder funded by the Abu Dhabi United Group Investment + Development Ltd (Adug), which owned the club. Sheikh Mansour bin Zayed Al Nahyan, of the Abu Dhabi ruling family, bought City in 2008.

City deny wrongdoing and will appeal against the verdict before Friday’s deadline. “Manchester City FC is both disappointed and surprised by the opinion of the Premier League commission,” a club statement read. “The club is innocent of the accusations made by the Premier League and a comprehensive body of irrefutable evidence exists in support of all of its positions, relating to this case. The club will therefore be relentless, and where necessary proactive, in any and all appropriate regulatory and legal forums.”

The club’s chief executive, Ferran Soriano, sent a video to City employees after the announcement in an attempt to reassure staff.

“The whole Premier League case against us is based on a single false accusation – that the owner’s personal money was somehow and secretly put into the club via some sponsors from Abu Dhabi,” Soriano said. “This is just not true. Irrefutable evidence has been provided to the Premier League commission that shows it could not happen and that it did not happen. This includes bank statements, money transfers, witnesses and everything necessary to categorically demonstrate the money did not come from the owner.”

The commission said City “clearly intended to circumvent the Premier League rules”. The club were also found guilty of failing to cooperate by trying to “stop and frustrate” the investigations, which began in December 2018.

A number of options are on the table regarding sanctions, including the stripping of titles, financial penalties and points deductions.

The Premier League’s chief executive, Richard Masters, described the case and decision as “the most significant” in its history. “The core decision establishes the facts of what happened at Manchester City during this period,” Masters said. “It details how the club systematically broke Premier League rules for nearly a decade.

“It also vindicates the Premier League’s decision to pursue this case against Manchester City. While the process to date has been both long and difficult, the league has remained determined that the facts be established independently.

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“It is a key responsibility of the Premier League to ensure that the rules, approved by the clubs themselves, are upheld to protect the integrity of the competition. It is paramount that the league remains competitive and fair for all clubs and for the fans.”

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Masters added: “There are elements of the case that remain to be decided, including, importantly, what sanction must follow for these breaches. Now we have the commission’s decision, we are committed to moving swiftly through the remainder of the process, to provide certainty for the league, our clubs and fans.”

The culture secretary, Lisa Nandy, said: “It’s incredibly serious, the Premier League report is incredibly stark. Manchester City have the right to appeal. This is causing a lot of concern for City fans and all fans of Premier League football.”