Sydney FC defender Sebastian Ryall has been suspended from the A-League after being charged with engaging in a sexual act with a 13-year-old girl in January 2008.

Ryall, who was 18 at the time, will plead not guilty to the charge.

Ryall, who was scheduled to leave on Saturday with the Australian under-20 team for a camp in the Netherlands, has also been suspended from international duty.

The Football Federation Australia says Ryall has been stood down for bringing the sport into disrepute.

Ryall was at the Melbourne Victory when the alleged incident occurred, but the Victory and Sydney FC say they were not aware of the charge.

FFA chief executive Ben Buckley said the matter involving Ryall was brought to the attention of the sport's ruling body and Sydney FC on April 23.

He said the matter had not been publicly disclosed until now because the issue had to be discussed with the player and his legal representatives, his club and the FFA's lawyers.

Ryall was sanctioned following a finding under clause 2.1 of the FFA's code of conduct that he had brought the game into disrepute through being charged with a criminal offence.

Ryall has pleaded not guilty to the charge and his lawyer said he would strenuously defend his innocence.

Buckley said the FFA had not formed a view of the player's innocence or guilt, stressing that the sanction was imposed on the basis that the criminal charge had brought the game into disrepute.

Ryall has seven days to appeal the FFA decision.

Sydney fully endorsed the sanctions imposed by the FFA.

"The Sydney Football Club, which has its own code of conduct, demands that only the very highest standard of behaviour is acceptable from all of its employees," said the club in a statement.

"This has been acknowledged by all of the club's players, coaches and staff.

"Given that the incident has resulted in a criminal charge and is presently before the courts, the club, its players, coaches and staff will not discuss any matters relating to the incident.

"Once the proceedings before the courts have been finally determined, the club will then make a formal statement."

- ABC/AAP