Thousands of pounds have been won in bets on Chelsea captain John Terry being substituted in the 26th minute of his final appearance at Stamford Bridge on Sunday.
The 36-year-old had arranged to go off against Sunderland in the minute matching his shirt number.
BBC Sport pundit Alan Shearer said it raised questions of integrity.
The FA has asked the betting companies involved for information on bets they received on the substitution.
One bookmaker said it had paid out on three bets, with one customer claiming he had been paid at 100-1 on a £25 stake.
He was given his winnings after being involved in an exchange on Twitter with the bookmaker as to whether the substitution occurred in 26th minute, when the board went up for Terry to come off, or the 28th minute, when he left the pitch.
The bookmaker said: “Clearly the send-off was planned for the 26th minute to commemorate JT – hence why we paid out.”
Another successful gambler, who staked £10, anonymously told the Press Association: “I only put money on this because I thought this is surely going to come in.
“I don’t normally bet. It’s only the second time I’ve ever placed a bet.
“I was surprised the odds were that high.”
Terry has admitted his 26th-minute farewell was his idea and he had agreed it with manager Antonio Conte.
Conte said: “He deserved this. He’s a legend of this club, not just this club but one of the best defenders in the world.”
Sunderland boss David Moyes said his side agreed to put the ball out to allow the substitution.
Former England captain Shearer said he was “not sure” about the timing of the guard of honour, adding on Match of the Day: “It was done with good intentions but I don’t think anything should be done that could undermine the integrity of the game.”
Culled from BBC