What is Financial Fair Play?
FFP was introduced by European governing body UEFA as an attempt to prevent clubs getting into serious financial difficulty by overspending. Clubs would have to operate within their means and meet break-even targets, while dealings would have to be transparent. This meant lavish expenditure - on transfer fees or player wages, for example - would have to be matched by income. It was also hoped it would encourage clubs to build towards sustainable growth on and off the field.
What are the regulations?
The regulations, which must be adhered to by all clubs participating in UEFA competitions, were drawn up in 2009 and introduced at the start of the 2011-12 season. Basically, clubs must balance their books over the course of three years. Investment in infrastructure, training facilities or youth training is exempt. Income needs to be generated by the club, including for example matchday takings, TV revenue, advertising, sponsorship, player sales and prize money.
What are the punishments for breaking the rules?
UEFA's club financial control body (CFCB), which monitors FFP, has a range of sanctions available to it for non-compliance. These range from warnings and reprimands to disqualification from a competition, exclusion from future competitions and being stripped of a title. Fines, points deductions and restrictions on the registration of players are also possible.
What are the criticisms of the system?
Some believe that FFP, while well intended, has had a negative effect because it restricts investment from honest donors who are not seeking profit. It is claimed these people might be investing - at a personal loss - purely for sporting success or to alleviate financial distress on clubs and it is unfair to prevent them. It is argued another unintended effect of this has been to preserve an existing hierarchy in football, limiting the ability of smaller clubs to grow and compete with bigger, richer clubs.
Which clubs have been punished?
Two of the most high-profile cases saw Manchester City and Paris St Germain, two clubs whose growth had been accelerated by huge new investment, punished in 2014. City accepted a settlement that included a £49million fine, a £49million limit on transfer spending for the coming season and a 21-man limit on Champions League squad size, instead of the usual 25. PSG received a similar punishment.
Why are City being investigated again?
UEFA has opened a fresh investigation into City following a series of new allegations about the club in the media, led by German magazine Der Spiegel. They allege that City's breaches of FFP around the same period run much deeper than UEFA realised at the time of the 2014 settlement and that deception was employed in their accounting methods. Among the allegations are claims that sponsorship deals were artificially inflated and claims over how image rights were paid. UEFA said in a statement: "The investigation will focus on several alleged violations of FFP that were recently made public in various media outlets."
How have City responded?
City have firmly denied the allegations, saying in a statement: "Manchester City welcomes the opening of a formal UEFA investigation as an opportunity to bring to an end the speculation resulting from the illegal hacking and out of context publication of City emails. The accusation of financial irregularities are entirely false. The club's published accounts are full and complete and a matter of legal and regulatory record."