The jury in the assault case against singer Cheryl Tweedy has been sent out to consider its verdict.
The judge said it was 'common ground' that Miss Tweedy used force
The Girls Aloud star has denied attacking nightclub worker Sophie Amogbokpa, saying she only punched her because she had been hit first.
The judge at Kingston Crown Court said on Monday there was no question that Mrs Amogbokpa had been injured in the incident.
He told the jury members they had to decide whether Miss Tweedy had acted in self-defence.
He said they would also have to decide whether she had referred to Mrs Amogbokpa as "black".
The charges against the 20-year-old singer, from Heaton in Newcastle, stem from an incident at the Drink nightclub in Guildford, Surrey, on 11 January.
Miss Tweedy is accused of having punched Mrs Amogbokpa and racially abused her after the toilet attendant tried to stop her taking some lollipops.
But the pop star has told the court she always intended to pay for the sweets and had hit 39-year-old Mrs Amogbokpa in self-defence, after the club worker punched her in the face first.
Judge Richard Haworth told the jury: "In this case, it is common ground that Cheryl Tweedy applied force to Sophie.
"She hit her.
"The issue is has the prosecution proved that what Cheryl did was unlawful?
"If you think that Cheryl was or may have been acting in lawful self defence, she is entitled to be found not guilty.
"You must ask whether Cheryl thought it was necessary to use
force to defend herself at all.
"This would not be the case if she was the aggressor, or acted in revenge or knew she did not need to resort to violence."
He added that Miss Tweedy would be entitled to be acquitted if the jury decided she had used reasonable force.
The jury also had to decide if any assault had been racially-motivated.
Judge Haworth said the crucial element was whether Miss Tweedy had called Mrs Amogbokpa a "black bitch".
He said: "The word black is essential to the case - bitch is not enough."
The judge also told the jury Miss Tweedy was "of good character", having no previous convictions and also being "an extremely hard worker".
Miss Tweedy denies racially aggravated assault occasioning actual bodily harm and an alternative charge of assault occasioning actual bodily harm.