Judge Lloyd asked for a pre-sentence report to be prepared by the Probation Service and warned Flanagan he would face a community penalty.
She said: "I need to be reassured that these circumstances will never be repeated again."
The defender, who wore a black suit and tie, was given unconditional bail until January 17, when he will be sentenced.
Liverpool-born Flanagan has made more than 50 first-team appearances for Liverpool FC since making his debut aged 18.
He spent the 2016-17 season on loan at Burnley after 20 months out due to injury and has featured only once in the first team this season as the Anfield club were knocked out of the League Cup by Leicester City. Sky more