In a statement issued on Saturday, Chelsea said it was clear Mr Johnson had "suffered unacceptably" and that it had "no desire to hide any historic abuse we uncover from view".
Chelsea said they were told by Mr Johnson's solicitors in 2014 of other unnamed victims including young boys, but said they had been unable to identify anyone else.
The club said an external review would examine whether they should have carried out "a more detailed investigation" when the allegations first came to light and why they did not report them to the Football Association and Premier League
Questions about how much the FA knew about issues of abuse, and when, mounted last night with reports claiming the organisation was warned on at least two occasions in prior decades about potentially inappropriate conduct towards young players.
It was previously claimed Chelsea paid off Mr Johnson, who claimed he was abused by ex-coach Eddie Heath.
Mr Johnson, 57, said he was paid £50,000 not to go public with allegations he was sexually abused by its former chief scout in the 1970s.
In a statement, Chelsea said: "The decision to have a confidentiality clause in this case has been subject to significant scrutiny."
The statement added: "In light of what we know now about the wide-scale abuse in football clubs in the 1970s and 1980s, it (the board) now believes that the use of such a clause, while understandable, was inappropriate in this instance."
Chelsea said on Tuesday they had appointed an external law firm to carry out a formal investigation into a former employee.
Mr Heath, who was the club's chief scout from 1968 to 1979, died before the allegations were made.
Former youth player Russell Davy, 50, has claimed he wrote to the FA in 1986 detailing allegations, but said he never received a reply
He waived his right to anonymity to the Sunday Mirror to say Mr Heath sexually abused him at Charlton Athletic Football Club when he was 15.
Earlier, former Chelsea star Alan Hudson, 65, also said it was "common knowledge" that Mr Heath "was a danger to us youngsters"
The developments come as the number of professional and non-league clubs named in allegations passed to police investigating child sex abuse so far climbed to 55, the Observer has reported.
A separate report for The Sunday Times also referred to an unpublicised 2002 report, commissioned by the FA about child protection in the game, that allegedly included a warning from a coach about "older females developing relationships with younger girls who are confused".
Reports have also surfaced via BBC Radio 4's Today suggesting a former employee of Southampton Football Club accused of abusing young players in the 1980s is still working in the game.
The programme said it understood the former staff member left Southampton after concerns were raised about his behaviour towards members of the club's youth team and said he was the fourth former player at the club who had come forward with allegations of abuse against the same man.
Southampton said they are "committed to working together" with Hampshire Police to investigate any historical allegations that may be brought to light in the area.
As the scandal continues to unfold, former Southampton player and England captain Alan Shearer, 46, also urged footballers to come forward with allegations of sexual abuse and said he was "shocked and deeply saddened" by recent revelations.
He appealed for victims to contact the NSPCC's football helpline, which has received 860 calls in a week.
Current captain Wayne Rooney, 31, also joined with figures, including women's captain Steph Houghton, in a video on "safeguarding" published for the FA and NSPCC.
West Midlands Police also said it was "investigating four historical allegations of child sexual abuse in football" and Kent Police said it had received reports of abuse within the county's football community.
So far 10 suspects have been identified as the scandal continues to grow, and Greater Manchester Police (GMP) said it was investigating reports from 35 people, with its inquiry growing on a "daily basis".
::The NSPCC hotline is available 24 hours a day on 0800 023 2642.
Source : PA
Source: PA