"Hundreds" of children may have been sexually abused by figures within football, former Tottenham and England player Paul Stewart has told the BBC.
Mr Stewart, who says he was abused by a coach for four years as a child, said the sport could face allegations on the scale of the Jimmy Savile scandal.
It comes as the NSPCC said more than 50 people had rung an abuse helpline within two hours of it being set up.
It was launched after four footballers spoke about being abused as children.
Former Crewe players Andy Woodward and Steve Walters, and ex-Manchester City player David White, as well as Mr Stewart have spoken out about abuse in the game after waiving their right to anonymity.
Football Association chairman Greg Clarke met Mr Woodward on Thursday to discuss the allegations. Mr Clarke told BBC Sport he wanted to ensure potential victims "felt safe" to come forward and "report terrible crimes".
Allegations of abuse within football surfaced last week, when Mr Woodward, 43, spoke publicly about his abuse by former Crewe coach and youth football scout Barry Bennell.
Two other players who say they were abused by Bennell will also waive their right to anonymity to speak on Friday's edition of the Victoria Derbyshire programme (09:00 GMT, BBC Two). They came forward after seeing Mr Woodward's interview.
On Thursday, Manchester City released a statement saying: "The club is aware of allegations that Barry Bennell had an association with Manchester City Football Club in the 1980s. As a result the club is currently undertaking a thorough investigation of any past links he might have had with the organisation."
According to the Guardian, an anonymous ex-footballer has also contacted police to say he was a victim of George Ormond - a former Newcastle United youth coach who was jailed in 2002 for offences against young footballers in the area.
A spokesman for Northumbria Police told the Guardian it had received an allegation of "historic sexual offences in Newcastle" and inquiries were ongoing.
Ormond was sentenced to six years in prison for sex offences against boys aged between 13 and 21 committed between 1974 and 1994.
The judge called him a "predatory abuser of young boys" who "used his position as a football coach to target vulnerable children" and "prevented disclosure by the power wielded over them as their coach".
'Abusers working together'
Mr Stewart, 52, who started his career at Blackpool and also played for Tottenham, Manchester City and Liverpool, first told the Daily Mirror an unnamed coach - not Bennell - abused him daily for four years up to the age of 15.
In an emotional interview with BBC sports editor Dan Roan, Mr Stewart said he believed there were "hundreds of victims" of sexual abuse who could come forward.
He also claimed:
- His abuser carried on offending for several years - and Mr Stewart has spoken to other victims
- His abuser knew Bennell and referred to him "almost to justify" the abuse
- The two coaches were "associated" with two other men "doing the same thing to kids"
- He "did not have evidence" of this collaboration, but said he believed "they were all in cahoots"
- He will be speaking to the police about his alleged abuse
Mr Stewart said he finally escaped his alleged abuser at the age of 15, signing for Blackpool, where he made his professional debut at 17.
"I was told that I had to do these things if I wanted to be a footballer," he said. "I only ever wanted to be a footballer as a child. He was threatening that he would kill my parents and my two brothers if I ever spoke out. I was just absolutely frightened."
Asked if he feared the allegations football is facing could be as big as the Savile scandal, he told the BBC: "Yes, I do, for sure. I would almost guarantee it as long as the victims are willing to come forward."
After his death, former DJ and television presenter Savile was found to be one of the UK's most prolific sexual predators.
Mr Stewart also spoke of the heartache of having to show his family the Mirror story before it was published, saying he had been unable to show his son and two daughters in person.
He added: "This has not been easy for me to do.
"But I felt that I needed to do this so other people will come out and with the hope that it may stop anyone who may be thinking of doing it again, in any walk of life - not just football."
He added: "The access to children at sport level is very easy and it is the perfect ground for them to prey.
"I do believe, in the days it was happening to me, it was a taboo subject, nobody spoke about it. It was brushed under the carpet if it was happening. I don't think there was anywhere to turn in those days."
Sports minister Tracey Crouch said the former players had shown "incredible bravery" to speak about the abuse.
Shadow sports minister Dr Rosena Allin-Khan warned the scandal could "seriously damage" UK football's reputation.
The NSPCC said callers to the abuse hotline had raised concerns about children now and in the past, and it expected "many more" to come forward.
Information from 20 callers would be passed to police, the children's charity said.
Manchester United and England captain Wayne Rooney, an NSPCC ambassador, said: "It's awful that some of my colleagues have suffered this way whilst playing the sport that I and they love. It's important that people know that it's OK to speak out, there is help available and that they don't need to suffer in silence."
'Courage and dignity'
Bennell was sentenced to nine years in prison in 1998 after admitting sexual offences against six boys.
He was a youth coach at Crewe Alexandra in the 1980s and 1990s and also had a close association with Stoke City and Manchester City.
Cheshire Police said 11 people had come forward since Mr Woodward spoke out, including fellow ex-Crewe player Mr Walters, 44, who says he was also a victim of Bennell.
Former Manchester City and England player Mr White, 49, says Bennell abused him between 1979 and 1980 while he was playing for Whitehill FC junior team in Manchester.
So far, Greater Manchester Police has not received any official reports of abuse but, in a statement, it said any victim should have confidence officers would treat complaints seriously and thoroughly investigate them.
On Thursday evening, police were seen at Bennell's address in Milton Keynes, removing a dog and some boxes.
Crewe's director of football Dario Gradi, who has been associated with the club for more than 30 years, released a statement saying he knew nothing about Bennell's abuse of young footballers until his arrest in the US in 1994.
"I knew nothing of his crimes before this time when he was employed by us," said Mr Gradi, who managed the club from 1983 to 2007 and for short spells afterwards.
"No-one at the football club knew of Bennell's crimes until his arrest in 1994 and his subsequent prosecution in the UK."
He said he would make no further comment until Crewe had carried out a review.
The Professional Footballers' Association (PFA) said it expected the number of people coming forward to rise.
PFA chief executive Gordon Taylor told BBC Sport: "There's always talk of hidden agendas and cover-ups. I can only speak from my own experience and the PFA's point of view. There's no cover up. It's our business to help players and resolve problems.
"We have seen an increase in the number of players to come forward. We can only do our best. There might be youngsters that this caused them not to have a career in the game. We need to encompass the whole lot."
Former FA chief executive, Mark Palios, told the BBC he was not surprised by the allegations.
He said it would be "naive" to suggest abuse has not gone on within the game.
"It's an issue like racism - you can't be complacent that you've actually dealt with it. But I'm pretty certain that the position today is far better than it was 20 to 30 years ago when it was pretty much un-regulated."
The FA's head of equality and safeguarding, Sue Ravenlaw, said the "courage and dignity" shown by the four footballers who have spoken out was "immense".
- The 24-hour NSPCC hotline number is 0800 023 2642