Police have detained six people after opening a second investigation into football match-fixing following allegations that a player rigged a Championship game in return for cash.
The National Crime Agency (NCA) confirmed this weekend that it was examining claims passed to it by the Sun on Sunday.
The newspaper said an undercover investigator met ex-Portsmouth defender Sam Sodje, who said he could arrange for footballers in the Championship to get themselves a yellow card in return for tens of thousands of pounds.
He also claimed he could rig Premier League games and even said he was preparing to fix matches at next year's World Cup in Brazil.
Filmed on camera, Sodje allegedly claimed he could arrange a booking in exchange for tens of thousands of pounds. The ex-Nigeria international also allegedly said he had punched an opponent during a game against Oldham in February in order to be sent off in exchange for a 70,000 pound payout.
Portsmouth spokesman Colin Farmery said: "If these serious allegations are true, then we are extremely shocked and saddened by them. 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 co-operate fully with any inquiry."
Also on film, former West Ham player Cristian Montano, now of Oldham Athletic, appeared to explain how he attempted to get a yellow card during a match against Wolves in return for cash.
Oldham said they were "aware of the allegations... surrounding Cristian Montano" and added: "The club will commence an immediate internal investigation to establish all the relevant facts of the case."
In a statement, the NCA said: "The NCA can confirm that the Sun on Sunday has passed material from its own investigation to the National Crime Agency.
"An active NCA investigation is now under way, and we are working closely with the Football Association and the Gambling Commission. We cannot comment further at this stage."
Football League chief executive Shaun Harvey said: "We treat any allegations of criminal activity in our competitions with the utmost seriousness. We would encourage anyone with any evidence to report it to the police. We will be giving our full assistance to the police during their investigation."
A statement from the Football Association said the governing body "was aware of the National Crime Agency investigation and is working closely with the NCA and other authorities" and that it would be making no further comment at this time.
The Professional Footballers' Association released a statement, which said: "The PFA is aware of the reports regarding allegations of match-fixing and other related activity.
"These allegations, if proven, unfortunately demonstrate the real issue football faces in terms of corruption and highlights the necessity of the work carried out by the PFA and other stakeholders in the game in educating players of these risks."
The NCA is already carrying out a separate investigation into an alleged international illegal betting syndicate. Six men were arrested earlier this month on suspicion of fixing league matches in England, with at least three players and an agent -- believed to be ex-Bolton player Delroy Facey -- being bailed.
Michael Boateng and Hakeem Adelakun, both 22, were charged this 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.
Information from the Press Association was used in this report.