LONDON • Sunderland's chief executive officer Margaret Byrne announced her resignation on Tuesday, after admitting to making "a serious mistake" in her handling of the Adam Johnson affair.

"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," she said in a statement after quitting her £663,000 (S$1.3 million)-a-year job.

Sunderland allowed the 28-year-old winger to continue playing for them for nearly a year despite knowing that he had admitted to kissing a 15-year-old girl.

He was sacked by Sunderland last month after he admitted to grooming the girl and one count of sexual activity with her. He is awaiting sentencing after being convicted last week of a more serious sexual activity offence.

The Premier League club said they had been unaware that Johnson intended to plead guilty to two of the charges facing him, but children's charities said he should have been suspended anyway.

In a statement, the Sunderland board said that it had accepted Byrne's resignation.

"Sunderland AFC acknowledges that Margaret's intentions have always been to act in the best interests of the club," it said. "However it has become clear through our own internal investigations that in this instance, decisions have been taken by Margaret in error."

Johnson, capped 12 times by England, has been told to expect a sentence of up to 10 years.

The club had stated that he told them he intended to deny all the charges against him.

"Contrary to what has been suggested, I did not understand that Mr Johnson intended to change his plea at trial or at all," said Byrne. "I was astounded when he did plead guilty. I accept that Mr Johnson should not have been permitted to play again, irrespective of what he was going to plead."

She said she had recommended to the club's board that Johnson be allowed to play on last year because he was fighting the charges and was "innocent until proven guilty".

The player was suspended by Sunderland in March last year after being arrested.

But two weeks later, after his bail was extended, the club lifted the ban. He played 28 games for the team before being sacked just prior to his trial last month, when he pleaded guilty.

"It was a serious error of judgment and I accept full responsibility for this," Byrne said. "Johnson's victim has endured a terrible ordeal and for that I am truly sorry."

AGENCE FRANCE-PRESSE, REUTERS