A ground-breaking documentary showing how the killer of schoolgirl Claire Tiltman was finally brought to justice will be aired this month.

The three-part series The Prosecutors, which was allowed unprecedented access to the Crown Prosecution Service (CPS) for the first time in its 29-year history, is scheduled to be screened by the BBC from February 24.

One of the investigations featured will be the murder of 16-year-old Claire in Greenhithe in January 1993. Viewers will see how the legal team and officers from the Kent and Essex Serious Crime Directorate built the case against Swanscombe milkman Colin Ash-Smith.

The knife-obsessed loner was already serving a life sentence for attacks on two other women when he was eventually convicted in December 2014 for killing Claire in a frenzied stabbing.

He was again sentenced to life imprisonment and with a minimum tariff of 21 years.

A final bid to appeal his conviction for the Dartford Grammar schoolgirl’s murder was thrown out by the Court of Appeal last year, effectively condemning the 47-year-old to spending the rest of his life behind bars.

The Prosecutors, produced by Gold Star, will also show behind-the-scenes footage filmed during Ash-Smith’s five-week trial at the Inner London Crown Court in Southwark.

The episode is scheduled to be aired on March 9.

One of the directors, Blue Ryan, said: “Until you are the victim of a criminal action directly, the work of the CPS – how they operate and the process involved – is not necessarily understood.

“The professionals and contributors in this series, through their honesty and first-hand testimony, allow the viewers to really understand the work of the CPS, what is involved in their attempt to bring people to account and their relationship with the public.”

It will be the third time Claire’s murder has featured in a TV series. Crimewatch showed a reconstruction of her killing in London Road at the time, as well as a special 10-minute feature after Ash-Smith’s conviction.