City are being investigated by the Premier League and Uefa for possible breaches of football's Financial Fair Play (FFP) rules after German magazine Der Spiegel published a series of stories last year based on leaked emails between club officials.
In an interview with Belgian magazine Sport and Strategy, Uefa's chief FFP investigator Yves Leterme said: "If what has been written about Manchester City is true, there might be a serious problem.
"This can lead to the heaviest punishment - exclusion from Uefa competitions."
Leterme, a former Belgian prime minister, explained that clubs must truthfully report their financial affairs to Uefa when they apply for a licence to compete in European club competitions.
The governing body's Club Financial Control Board then randomly checks those figures and club accounts are examined by internal and external experts.
"If the (Der Spiegel) information is correct, this goes against the truthful reporting of financial affairs," added Leterme.
According to the German magazine, only £8m of City's claimed £67.5m sponsorship deal with Etihad actually came from the airline in 2015, with the rest coming direct from their owner Sheikh Mansour via his Abu Dhabi United Group.
A member of Abu Dhabi's royal family, Sheikh Mansour is deputy prime minister of the United Arab Emirates and his half-brother Sheikh Khalifa is the UAE's president and emir of Abu Dhabi.
The magazine also alleged that City backdated and inflated several other deals from their predominantly UAE-based sponsors to meet FFP requirements, hid payments to former manager Roberto Mancini and artificially reduced their image-rights costs.
The Premier League champions, who closed the gap to this season's leaders Liverpool with a thrilling 2-1 win over the Merseyside club on Thursday, have described the leaks as an attempt to smear their reputation but have not challenged the authenticity of the emails.
The Premier League is understood to be investigating the matter, too, and is cooperating with Uefa's investigators, who are also looking into similar allegations related to French clubs Monaco and Paris St Germain.
CIty and PSG were both given £49m fines by UEFA for breaching FFP rules in 2014, as well as having restrictions imposed on the size of their Champions League squads and transfer spends for two seasons - but two thirds of the sums were later returned to the clubs for apparently complying with the terms of the conditional punishment.
A second offence, however, would be viewed much more seriously, making a sporting sanction, a ban, more likely.
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