Margaret Byrne has resigned as Sunderland AFC's chief executive over her handling of the Adam Johnson child abuse case.
Johnson, 28, is facing a jail sentence of up to 10 years after being found guilty on March 2 of sexual activity with a 15-year-old girl.
In a statement released on Tuesday afternoon, Byrne said: "I recognise that, as CEO, my involvement with Mr Johnson and the decision to allow him to continue to represent Sunderland was a serious mistake."
Johnson was first suspended in March 2015, two weeks after his initial arrest, but it was lifted 16 days later.
He continued to represent the club until he was sacked on 11 February 2016 – the day after he pleaded guilty to one charge of grooming and one charge of sexual activity with a child.
Byrne's announcement comes amid growing pressure on Sunderland to explain why the club had allowed the former Premier League footballer to continue playing despite already knowing he had admitted to kissing and grooming the underage fan.
Byrne, a former criminal lawyer, is thought to have played a key role in recruiting Orlando Pownall as Johnson's barrister.
The court heard Byrne, who had been chief executive since 2011, had organised for the pair to meet on 4 May 2015 at Sunderland's training ground, the Academy of Light.
It is believed this is where she provided Pownall with copies of Johnson's police interview, in which he admitted kissing and grooming the victim.
The interview also contained details of 834 sexually charged WhatsApp messages between Johnson and the 15-year-old girl.
Sunderland said Byrne was "accountable for the actions taken by the club in relation to Mr Johnson".
Although Byrne sacked Johnson once he had pleaded guilty, the club admitted the decision not to suspend the footballer for a second time pending the outcome of the trial was "wrong".
A club statement said:
Sunderland AFC acknowledges that Margaret's intentions have always been to act in the best interests of the club, however it has become clear through our own internal investigations that in this instance decisions have been taken by Margaret in error.
Whilst swift and decisive action was taken to terminate Mr Johnson's employment upon his guilty plea, decisions taken prior to this, including the decision not to suspend him for a second time pending the outcome of the trial, were wrong.
In light of what has been acknowledged by Margaret as a serious error of judgment on her part, we have undertaken a full review of the club's decision-making processes to ensure that there can be no such mistakes in the future.
The club apologised for the "regretful situation" and said "lessons have been learned":
Throughout this deeply regretful situation, we recognise that one devoted young fan and her family have been very badly let down, first and foremost by Mr Johnson and his despicable actions, but also by the club they support. We are so very sorry for this.
Mr Johnson lied to the club; he also lied to our fans and they have every right to feel aggrieved by this. Lessons have been learned and we hope that the club and its fans can move forward from this together.