Police abandoned negotiations in favour of direct action at Lancaster Magistrates Court after the hostage-taker, Terrence Clifton, 28, became "increasingly distressed".
Mr Harper was praised by the police for remaining calm.
At about 10am three unarmed police of the Operation Support Unit rushed into the interview room in the basement of the court where Mr Harper is understood to have been threatened with three "weapons" - a pen, a radio aerial and a metal binder from a folder.
David Smith, Assistant Chief Constable of Lancashire police, said: "They overpowered the prisoner. The successful outcome was that Mr Harper was released without harm to either of them. Mr Harper was shaken by his ordeal.
"His demeanour during his ordeal has been exemplary. I wish to pay tribute to the calm way he disported himself. This was a factor in bringing this to a successful resolution.
"It was the professional judgement of the officers to go and overpower the prisoner."
The Criminal Law Solicitors' Association yesterday said it "deplored the lax security measures that allowed a client accused of violent criminal offences to take hostage a senior solicitor who was advising him."
Mr Clifton was winded during the release. He is accused of murdering garage owner Antonio Marrocco, 48, and Paul Sandham, 28, a business partner, in Morecambe in January 1995.
Mr Marrocco was found battered and stabbed to death in his garage. Hours later Mr Sandham's body was found in a field. He had been stabbed 30 times.
Mr Clifton, who had been brought from Walton Prison, Liverpool, is understood to have been searched at the prison and the court.
Meanwhile Britain's longest running house siege appeared to be coming to an end yesterday in Hull, Humberside. Armed police continued to surround a house where Steven Wood, 39, a convicted kidnapper armed with a pistol, has been evading arrest for 12 days.
Mr Wood, who has fired three shots in the house during the siege, released the second of his two women hostages on Thursday.
The 40-year-old woman, a former girlfriend, was undergoing counselling yesterday after her 11 day ordeal on the Orchard Park Estate.
Her 20-year-old daughter was set free last Saturday night, a week after she had been taken hostage.Reuse content