Metropolitan Police sergeant Delroy Smellie, 47, was due to go on trial charged with common assault by beating Nicola Fisher on April 2 last year.
But City of Westminster Magistrates' Court heard Ms Fisher was "unwilling" to attend court and prosecutors have been struggling to contact her.
Prosecutor Nick Paul said the case could go ahead using a witness statement she gave to the Independent Police Complaints Commission (IPCC) alongside video and photograph evidence.
But 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 is "not in the interests of justice".
The court heard that Ms Fisher's former boyfriend Gavin Shepherd, who was due to be a witness, also failed to attend court.
The judge adjourned the case for prosecutors to make further inquiries about his whereabouts. The trial was due to be heard without a jury.
The court heard that Ms Fisher was "unwilling" to attend but has been represented by Max Clifford, and given media interviews and evidence to the Home Affairs Committee.
Mr Paul said Ms Fisher knows she is required in court as a witness summons was posted through her door after colleagues contacted her by telephone at the weekend.
The prosecutor said her former boyfriend, who witnessed the confrontation from a short distance, had "gone off the scene" and has only just been contacted.
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.
Ms Wickham was told Ms Fisher was planning to see a doctor this afternoon and an application may be made for an adjournment depending on her "fitness to attend".
The judge said there was evidence that Ms Fisher has been "nervous and wary" of the court process since the beginning of February and that she has doubts in the abilities of prosecutors.
Ms Wilding said a doctor's note intended for work was "inadequate" and she would require disclosure of all medical notes about the witness's condition.
She said: "We submit that a line has to be drawn. This is too little and too late and it is inadequate."
The defence solicitor added: "The defendant, as with all others, deserves some finality about the state of proceedings in a case he has to meet."
Evidence of the clash outside the Bank of England during a vigil following the death of Ian Tomlinson was revealed during several hours of legal argument.
The court heard the incident was caught on video and posted on the internet site YouTube, leading to a large amount of media interest. Several photographs also emerged.
The footage showed 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.
Mr Paul said Ms Fisher may be reluctant to attend court because the defence may "attack her character" by focusing on her lifestyle and background.
Speaking about the prosecution 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.
"To put it in a nutshell, that material calls for an explanation and that explanation is contingent on the factors that Sergeant Smellie perceived them to be at the time.
"You will see from the opening note in which I summarise the interview, essentially his case is he became aware of this woman, he pushed her back.
"She came forward again, he delivered the clearance swipe and she came forward again and that is the extent of his contact with her prior to the confrontation between them, which is graphically demonstrated in material before the court."
Mr Paul added that the officer's defence team want to question Ms Fisher's reliability as a witness.
He said: "They, without beating about the bush, wish to explore with her the fact she was aggressive, vocally confrontational and provocative in the moments up and until the physical confrontation.
"She does not describe herself as doing anything other than being caught up in the interface between police and the crowd when Sgt Smellie tells the crowd to get back.
"What the defence seek to do is establish that she was effectively trying to seek a confrontation with the police and in particular with Sgt Smellie."
Mr Paul said the judge must decide whether her demeanour and any provocation could justify the officer's actions.
Ms Wilding said Ms Fisher is clearly an "unwilling and reluctant witness" and described the circumstances as "highly unusual" because of the interviews she gave.
She said the case focused on Smellie's use of his baton to hit her thigh as the "back-hand strike is not said to be unlawful".
The solicitor said: "Mr Smellie is charged with an offence, the defence to which is he was acting lawfully in self-defence both of himself and colleagues who were posted with him.
"The question arises whether the degree of force used against a person was reasonable in the circumstances."
Ms Wilding said her client does not accept that the video "speaks for itself" and said questioning Ms Fisher is the "essence" of his defence.
The legal argument was continuing.
