He was forced to spend 20 months off front line duties, under suspicion of assaulting a drunken thug who had tried to butt him.

But today Sgt Bob Woodward was finally free to continue his career, after the investigation ended in farce and the case was thrown out in minutes by a crown court judge.

A not guilty verdict was announced after the prosecution offered no evidence against Sgt Woodward at the start of his trial yesterday.

The case collapsed because the alleged victim, criminal Wayne Pearson, is on the run after skipping bail for another offence. The nightmare started for the 6ft 8in father-of-three from Hednesford in July 2007 when he brought Pearson into Cannock police station after arresting the 23-year-old for an alleged breach of bail.

The suspect suddenly tried to butt the 52-year-old officer as they stood together in the custody suite, Birmingham Crown Court heard during legal arguments about the case yesterday.

Sgt Woodward blocked the blow and pushed his attacker onto a desk where Pearson’s front tooth was chipped as his face hit the formica top and a bunch of keys. Pearson did not make a formal complaint but Staffordshire Police launched an investigation following an anonymous tip-off.

Details of their findings were passed on to the CPS which decided to prosecute Sgt Woodward. He was taken off front line duties early last year when formally summonsed for assault and has since been doing other work or been on sick leave.

Meanwhile, Pearson, of Chadsmoor, was jailed for an unrelated matter and is now on the run on suspicion of being involved in the recent glassing of a man in a Staffordshire pub. Mr Zaheer Afzal, prosecuting, told Judge Sean Morris: “Regrettably our main witness is not here today, and we have not been able to find him.”

He said he had been arrested over a glassing, adding: “He was bailed without charge and told to return on February 9. He did not re-appear. He is now wanted.”

Mr David Mason, defending Sgt Woodward, said he found it “staggering” that the case had taken so long to get to court.

He told the court: “The officer thought he was going to be head butted and was using reasonable force to protect himself from a clearly drunk, violent and aggressive man.

“I am representing a police officer of good character with 30 years’ service who has been waiting a long time for this while the main prosecution witness has been involved in another allegation of violence.”

The prosecution said two of their other witnesses would not be called because their evidence “benefited Mr Woodward’s defence”. And they admitted that a DVD showing the incident taken from a camera in the police station had not been included in exhibits for the case.

Judge Morris told Sgt Woodward, who retires from the police in April: “This is now over, and you are free to go.”