Three people are in police custody after an ex-Portsmouth player told an undercover reporter he had been involved in fixing football matches.
Sam Sodje was filmed by the Sun on Sunday describing how he punched an opponent in a League One game to get a red card in exchange for £70,000.
He also said he arranged for another player to be paid £30,000 for getting a yellow card in a Championship match.
Portsmouth Football Club said it was “shocked and saddened” by the claims.
The three people being held have not been named.
In the video, which was filmed secretly, Mr Sodje also claimed he could rig Premier League games and was prepared to fix matches at next year’s World Cup.
In a statement from Portsmouth Football Club, 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 co-operate fully with any inquiry.”
Mr Sodje was sent off in the 50th minute of a League One match while playing for Portsmouth against Oldham Athletic on 23 February.
The Sun on Sunday also reported that an Oldham player, Cristian Montano, apologised to Mr Sodje after failing to get a yellow card in the first half of a match against Wolves on 22 October.
And it reported that Mr Sodje’s brother Akpo, a striker at Tranmere Rovers, had said he would be prepared to be paid to be booked.
Oldham Athletic 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.”
Tranmere Rovers FC said: “Tranmere Rovers Football Club are aware of Sunday newspaper allegations regarding Akpo Sodje. As this is an ongoing investigation the club will make no further comment at this time.”
The National Crime Agency, which is investigating the claims, said: “An active investigation is now under way and we are working closely with the Football Association and the Gambling Commission.”
The Football League said it took claims of criminal activity “extremely seriously” and would assist police.
Football League chief executive Shaun Harvey encouraged anyone with any evidence to report it to the police.
Mr Harvey added: “We treat any allegations of criminal activity in our competitions with the utmost seriousness.
“We will be giving our full assistance to the police during their investigation.”
The Gambling Commission and the Football Association both said they were aware of the investigation and were working with the NCA.
Guy Whittingham, who was manager of Portsmouth at the time, said the claims brought back memories of the match in which Sam Sodje was sent off.
He said: “Sam came racing over – and I didn’t actually see the incident at the time itself – but then you see it in the cold light of day afterwards and you see what he’s done and you can’t fathom out why he’s done it.”
Former Oldham striker Andy Ritchie told BBC News he was “dismayed” at the allegations.
He said: “They are only allegations at the moment but if you’re sent off in a game you’re letting your players down, you’re letting the team down because more often than not 10 men lose a game of football.
“If you’re in a tough situation, maybe a relegation dogfight, you could lose those three points – that could cost you a place in that league.”
In a separate NCA investigation into an alleged international betting syndicate, two non-league footballers were earlier this week charged with conspiracy to defraud.
Michael Boateng and Hakeem Adelakun, both 22, play for Whitehawk FC, based in Brighton, in the Conference South.
Two other men, Singapore national Chann Sankaran, 33, and Krishna Sanjey Ganeshan, 43, who has dual UK and Singapore nationality, were charged with conspiracy to defraud in November.
The pair have been bailed and are due to appear at Birmingham Magistrates’ Court on 11 December.