The Premier League is to ask Cardiff for their observations after The Sun reported that owner Vincent Tan offered his players an extra bonus for avoiding relegation.
Regulations state that any bonuses must be officially lodged before the start of the season and not offered on an 'ad hoc' basis.
The Bluebirds are unlikely to face disciplinary action, however, after a club spokesman said Tan had been unaware of the rules and has since withdrawn the offer.
League rules state: "Full details of a player's remuneration including all benefits to which he is entitled whether in cash or in kind shall be set out in his contract. The terms of a contract between a club and a player shall be strictly adhered to.''
Tan reportedly told his players before the 1-0 defeat by Tottenham on Sunday they would share in a 3.7 million-pound bonus payment if Cardiff retained their place in the top flight.
Such agreements are commonplace as Premier League status means more income for clubs, but they must be made clear in players' contracts or a bonus schedule before the start of the campaign.
A spokesman for Malaysian businessman Tan told The Sun: "Vincent didn't realise it was against the regulations. He has since rescinded the offer, which he made only with Cardiff's best interests at heart.''
The Bluebirds are locked in a battle to stay in the top flight as they sit three points adrfit of safety in 19th position.