Manchester City are claiming the Premier League has treated them unfairly in assessing their commercial income by relying on the analysis of a data company that also works for their rivals. The Guardian has learned the league’s scrutiny of the fair market value of City’s deals was undertaken by Nielsen Sports, a global data and media valuation firm that has contracts with several top-flight clubs. City are understood to have raised the Premier League’s use of Nielsen in their legal battle over associated party transactions (APT), in which clubs strike sponsorship or revenue deals with businesses linked to their owners, which was heard in private last month. A decision on the landmark case is expected soon, although it is unclear when it will be made public. The club and the Premier League are also preparing to lock horns in an era-defining legal hearing in November, with the winners of six of the past seven league titles accused of 115 breaches of financial fair play rules. Both parties declined to comment on either case when contacted. City have denied wrongdoing. The Premier League appointed Nielsen to help police its APT rules when the regulations were introduced in 2021 in a move approved by the clubs. It is understood the Nielsen team working for the league on APT rules is ringfenced from the rest of its business, an operation with eight offices around the world. City have brought a hugely significant legal challenge against the Premier League’s introduction and enforcement of APT rules, leading to an 11-day arbitration hearing. The club voted against APTs when they were first tabled three ago and again when the rules were tightened in February this year. It is claimed by City that the regulations, designed to ensure sponsorship received from companies with links to clubs’ owners are of fair market value, are unlawful because they contravene competition law. If successful, the club will demand financial damages from the Premier League for perceived losses from sponsorship deals blocked after analysis by Nielsen. City are also understood to be arguing the Premier League’s APT rules are far more restrictive than Uefa’s and want them relaxed. The unprecedented legal action has sparked a civil war in the top flight, with Newcastle, Chelsea and Aston Villa sympathetic to City but Arsenal, Tottenham and Liverpool strongly opposed to their claim. Legal documents from City allege the rules were designed to “discriminate against Gulf owners”. That bold claim is based on the fact that APTs were brought in as a response to the Saudi-backed takeover of Newcastle in 2021. City posted a Premier League record revenue of £712.8m last November on the back of winning the treble, an increase of almost £100m on the previous 12 months, and their commercial income has led to eyebrows being raised at other clubs given the size of their global fanbase compared with that of Manchester United and Liverpool. Three of City’s biggest sponsors have close links to their Abu Dhabi owners, including the shirt and stadium sponsors Etihad Airways, Etisalat and Experience Abu Dhabi. The new legal claim from City was filed in February, with the Premier League informing its clubs of the matter in March. In another element forming part of the legal challenge, City make an explosive claim about the Premier League’s well-established voting rules. The requirement that 14 of 20 clubs must vote in favour of any proposal for it to be adopted has long been hailed as one of the competition’s strengths, but in their legal documents City claim the system preserves “the tyranny of the majority”. Manchester City, the Premier League and Nielsen declined to comment.