Six people have now been arrested in connection with football match-fixing allegations.
Police launched an investigation after accusations that a player rigged a Championship game for cash.
The National Crime Agency (NCA) confirmed that it had acted on information passed on by the Sun on Sunday newspaper.
The tabloid reported that an undercover investigator met ex-Portsmouth player Sam 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.
The 34-year-old 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, the newspaper reported.
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 that Oldham Athletic player Cristian Montano 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 Montano. 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 underway 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."
The NCA is already carrying out a separate investigation into an alleged international illegal betting syndicate.
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.
Speaking about the latest reports, Football League chief executive Shaun Harvey said: "We treat any allegations of criminal activity in our competitions with the utmost seriousness."
In a statement, the Football Association said: "The FA is aware of the National Crime Agency investigation and is working closely with the NCA and other authorities. We will make no further comment at this time."
A Gambling Commission spokesman said: "The Gambling Commission has responded quickly in supporting this ongoing National Crime Agency investigation and continues to liaise with both the NCA and the Football Association."
Meanwhile, Marcus Gayle, manager of Conference South team Staines Town, has claimed his players were offered match-fixing bribes just three weeks ago.
The former Wimbledon and Watford player told BT Sport he was furious when he discovered the approach, which was made to one of his players by telephone.
Gayle said: "We reported the incident straight away to the FA. That shows we have done the right thing. Our players are absolutely furious.
"I never thought match-fixing was possible but now I have changed my mind for obvious reasons, now I am convinced it's all over the place, at least that's how it appears to be to me.
"After what has happened at my club I am angry and I want to stand up for the integrity of the game, even if that means standing up alone, so be it.
"As a club I believe we have done the right thing reporting it to the FA and I am sure they will be passing on that information to the police. We reported it to the FA 24 hours after I found out about it.
"It does worry me just how much is going on in the game at the moment, and I am sure it is far more than people think."
The Professional Footballers' Association (PFA) said in a statement: "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.
"We take the issue of integrity very seriously and will continue in our efforts to eradicate this evil from our game.
"In terms of these specific reports, due to the ongoing investigation by the National Crime Agency, we are unable to comment further at this time."