Sunderland chief executive Margaret Byrne has resigned for allowing Adam Johnson to continue playing for the Premier League club after he was charged with two counts of sexual activity with a 15-year-old schoolgirl last May.
Johnson remained a first-team regular for Sunderland until last month when he was sacked after pleading guilty on one count of sexual activity with a child.
The 28-year-old now faces a jail sentence after being found guilty last week.
Sunderland claimed they were surprised by the former England international's guilty plea, yet Byrne today admitted she was aware that Johnson had kissed the victim but had neglected to inform the Sunderland board.
"I accept that Mr Johnson should not have been permitted to play again, irrespective of what he was going to plead," said Byrne, who had been chief executive since 2011.
"It was a serious error of judgment and I accept full responsibility for this. The club's board was made aware of the broad nature of the allegations, which were a matter of public knowledge, but not of the detail that I was personally privy to."
Sunderland agreed Byrne had made a "serious error of judgment" and had undertaken "a full review of the club's decision-making processes to ensure that there can be no such mistakes in the future".
Byrne added: "I recognise that, as CEO, my involvement with Mr Johnson and the decision to allow him to continue to represent the club was a serious mistake.
"I sincerely regret that this error has impacted on the victim, the club, its supporters and all those affected in such a devastating manner.
"Sunderland AFC has a long and proud history. To see the criticism levelled against it in the last week, and the effect that has had on its staff and supporters in light of this decision, has hurt me deeply.
"To this end and for the long-term good of the club and its staff, I have stepped down as CEO with immediate effect."