Police have arrested six people in connection with a second investigation into football spot-fixing following allegations former Portsmouth defender Sam Sodje rigged a Sky Bet Championship game for cash.
The National Crime Agency (NCA) confirmed it had acted on information passed on by the Sun on Sunday newspaper.
The tabloid reported an undercover investigator met Sodje, who allegedly said he could arrange for footballers in the Championship to get themselves a yellow card in return for tens of thousands of pounds.
Portsmouth FC spokesman Colin Farmery said: "If these serious allegations are true then we are extremely shocked and saddened by them, as match-fixing of any type goes to the heart of the integrity of the game.
"The player in question no longer plays for the club and we have not been contacted by the authorities, but of course we would cooperate fully with any inquiry."
It was also reported Oldham Athletic player Cristian Montaño failed to get a yellow card in return for money in a match against Wolverhampton Wanderers on October 22 and offered to take part in another rigging incident.
Oldham said in a statement: "Oldham Athletic Football Club has been made aware of the incident and allegation surrounding one of its players, Cristian Montaño. The club will commence an immediate internal investigation to establish all the relevant facts of the case.
"The club is co-operating with other agencies in this matter and cannot comment on specific facts at this stage. The club will not be issuing any further statements at this time."
An NCA spokeswoman said: "An active NCA investigation is now under way and we are working closely with the Football Association and the Gambling Commission.
"Six people are in custody and are being questioned by NCA officers. We cannot comment further at this stage."
Michael Boateng and Hakeem Adelakun, both 22, were charged last week with conspiracy to defraud contrary to common law.
The pair, who played for Conference South club Whitehawk FC in Brighton, were dismissed by their club and will appear in court on Wednesday.
Two other men, Chann Sankaran, 33, from Hastings, East Sussex, and Krishna Sanjey Ganeshan, 43, from Singapore, were charged last month with plotting to defraud bookmakers and will also appear in court this week.