A star witness lying under oath to protect Babes in the Wood murderer Russell Bishop in 1987 was a “nightmare” scenario and had a “fatal effect” on his trial, a senior prosecutor says.

Jennifer Johnson, Bishop’s girlfriend at the time, stood up in court and gave false evidence about a crucial piece of evidence: a blue sweatshirt found near the scene.

The trial followed the high-profile murders of schoolgirls Nicola Fellows and Karen Hadaway, whose bodies were found in a woodland den in Brighton, East Sussex, in October 1986.

Bishop was found not guilty, but in the 34 years that followed the Crown Prosecution Service (CPS) saw Johnson’s lies as “unfinished business”, a senior prosecutor told the PA news agency.

Convicted by majority verdict of perjury and perverting the course of justice on Monday, 55-year-old Johnson could now face prison time.

She claimed she acted under duress when she gave evidence at Bishop’s first trial, saying she felt “intimidated and frightened” by his family, but was found guilty at Lewes Crown Court.

Senior Crown Prosecutor Libby Clark said Johnson’s decision to lie at the 1987 trial was a “nightmare” for the prosecution.

The whole case hinged on a blue Pinto sweatshirt found near the scene of the murders, she told PA.

“There was some limited scientific evidence in terms of fibre transfers but the evidence that Jennifer Johnson gave was really crucial potentially to establishing that that sweatshirt belonged to Russell Bishop.”

She had initially told police the now-infamous garment belonged to Bishop, but when it came to the trial she told jurors that it did not.

“It had a hugely detrimental effect, well as we know it was a fatal effect, on the trial of Russell Bishop”, Ms Clark said.

Bishop was acquitted of the murders and it took more than three decades and a change in the law before he was found guilty at a retrial in 2018.

The devastating impact of the original not guilty verdicts was highlighted in 1990, when Bishop kidnapped and sexually assaulted another young girl, leaving her for dead.

He was found guilty of attempted murder and sexual assault and jailed for life for that crime.

Ms Clark said: “Although she’s not responsible for the actions of Russell Bishop in 1990, the acquittal did give him the opportunity to go on to commit those other offences.”

Johnson’s convictions for perjury and perverting the course of justice this week mark the end of a long road, not just for the CPS but also the families of Nicola and Karen.

Ms Clark said: “It was unfinished business and naturally we’re very pleased that it’s resulted in a successful conclusion: a guilty verdict from the jury.

“What it must have been like for the families, who knew Jennifer Johnson, to have sat there (in 1987) expecting her to say one thing and then to say something which was completely contrary to the statement that she’d given to the police.

“So they’ve had decades really of despair until the conviction of Bishop in 2018, and must have thought an awful lot about what Jennifer Johnson’s role in all of that was in terms of giving the false evidence before the court.”

The sheer mass of material to go through presented a challenge for prosecutors pursuing charges against Johnson, Ms Clark said.

The CPS set about “reconstructing” her life with Bishop both before the 1987 trial and afterwards in an effort to demonstrate to the jury that she gave the evidence she did because she wanted to, not because of threats made against her or her immediate family.