Sgt Smellie is charged with common assault
The trial of a police officer accused of assaulting a G20 protester was halted after the alleged victim failed to turn up to give her evidence.
Metropolitan Police sergeant Delroy Smellie, 47, was due to go on trial charged with common assault during protests in April 2009.
Nicola Fisher was unwilling to attend Westminster Magistrates Court and could not be contacted, the court heard.
The case was adjourned so prosecutors can try to find Ms Fisher.
Prosecutor Nick Paul argued the witness statement she gave to the Independent Police Complaints Commission (IPCC) alongside video and photograph evidence could be used in the trial.
But Sgt Smellie's solicitor Lisa Wilding said it would be unfair to use Ms Fisher's important evidence without being able to question her about it.
District Judge Daphne Wickham ruled that Ms Fisher's witness statement could not be read to the court as it was "not in the interests of justice".
'Nervous and wary'
The court heard that Ms Fisher's former boyfriend Gavin Shepherd, who was also due to be a witness, had failed to attend court.
Ms Fisher had a witness summons posted through her door after colleagues contacted her by telephone at the weekend so knew she was expected in court, Mr Paul said
The court heard that Ms Fisher claims to have been suffering from depression and has shown a doctor's note to an official from the IPCC at her home in Brighton.
She was planning to see a doctor and an application may be made for an adjournment depending on her "fitness to attend".
The judge said there was evidence that Ms Fisher had been "nervous and wary" of the court process since February and that she has doubts in the abilities of prosecutors.
However, Ms Wilding said a doctors note intended for work was "inadequate" and she would require disclosure of all medical notes about the Ms Fisher's condition.
She added: "The defendant, as with all others, deserves some finality about the state of proceedings in a case he has to meet."
Caught on video
The alleged incident took place on the second day of the protests, which were held to coincide with the gathering of world leaders in the city.
The incident was caught on video and posted on the internet site YouTube. It showed Sgt Smellie shouting at Ms Fisher, pushing her back, striking her with the back of his hand and hitting her across the thigh with his baton.
Speaking about the case Mr Paul said: "The issue in this case is whether or not when Mr Smellie struck Ms Fisher he was doing so in self-defence.
"The evidence that we submit fully and really extensively covers the relevant circumstances.
Mr Paul added that the officer's defence team want to question Ms Fisher's reliability as a witness and wish to explore with her the fact that she was aggressive "in the moments up and until the physical confrontation".
Ms Wilding said Ms Fisher was an "unwilling and reluctant witness" and described the circumstances as "highly unusual" because of the interviews she gave.
She said the case focused on Sgt Smellie's use of his baton to hit her thigh as the "back-hand strike is not said to be unlawful".