Yvonne Fletcher, 25, was shot while policing a demonstration against Muammar Qaddafi outside the Libyan People's Bureau in St James's Square, London, on April 17, 1984.
Retired police officer John Murray, 66, is bringing a civil claim for a nominal amount of £1 against Saleh Ibrahim Mabrouk, who was an aide to the former Libyan leader, in a bid to air the "full evidence" surrounding his friend's death.
Mr Murray's lawyers previously hoped key material that has been kept secret for national security reasons would be presented in court for the first time.
Speaking before a three-day hearing at the Royal Courts of Justice in London that starts on Wednesday, Mr Murray said if he won his case it could result in the Crown Prosecution Service reviewing a decision not to proceed with a criminal prosecution.
"Once the full evidence is heard in open court and is made public, I think a lot of people will be surprised," he said.
"Everybody knows about the murder of Yvonne Fletcher. What people don't know, and probably don't understand, is what went on beforehand and what has gone on afterwards, and therein lies a real story."
Mr Mabrouk, who denies wrongdoing, was arrested in 2015 in connection with Fletcher's death, but two years later the Metropolitan Police said that, while detectives could identify those responsible, charges could not be brought against them.
This was because crucial evidence had been kept secret to protect national security.
Mr Mabrouk was allegedly inside the Libyan embassy on April 17, 1984, when a gunman opened fire from inside the building as anti-Qaddafi demonstrators and his supporters gathered outside.
Mr Murray, from Chingford, east London, who has suffered from post-traumatic stress disorder, cradled his colleague as she lay dying, promising he would find her killers.
"I can recall the events of that day by the minute. It's still fresh in my mind. I think about it all the time," he said.
"I was with her in the ambulance on the way to hospital.
"She was still conscious, she was still alive.
He said it was then that he promised to find out who had "done something to her" and why.
"It's taken a lot of time and a lot of money to get here," said Mr Murray, who is due to give evidence at the hearing.
"It's something we shouldn't have had to do because I do think criminal charges should have been brought.
"What I fought for for all this time is for the full evidence to be heard in front of a judge, and let the judge decide.
"That's all we want, we want justice at the end of the day.
"That's what she and all her supporters deserve."