The prosecution of schoolgirl killer Colin Ash-Smith is to feature in a ground-breaking television series.

For the first time in its 29-year history, the Crown Prosecution Service (CPS) allowed cameras to document its work with unprecedented access to several cases, including the historic murder of Claire Tiltman.

The 16-year-old was stabbed to death by Ash-Smith in a frenzied attack in an alleyway off London Road, Greenhithe, in January 1993.

Former milkman Ash-Smith, who lived in Swanscombe, was a suspect for many years but could not be linked at the time to the teenager’s murder.

But a cold case review by the Kent and Essex Serious Crime Directorate in 2013 led to the decision by the CPS to prosecute, and the 47-year-old was finally convicted and jailed for life, with a minimum tariff of 21 years in December last year.

He was effectively condemned to spend the rest of his life behind bars when three of the country’s top judges rejected an appeal bid last month.

How the police and lawyers built their case against Ash-Smith will feature in The Prosecutors, a three-part series to be screened by the BBC in the new year.

Production company Gold Star worked at close quarters with the CPS, and the work of award-winning directors Sara Hardy and Blue Ryan includes behind-the-scenes footage shot during the five-week trial.

Blue 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.

"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" - Director Blue Ryan

“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."

Other cases to be featured in the series include a death by careless driving, murder investigations, allegations of indecent assaults and the prosecution of an organised gang that targets cash machines.

During the making of The Prosecutors, Sara and Blue became close to the women who set up the Justice for Claire campaign, four of whom attended every day of legal proceedings against Ash-Smith.

They were given a preview of the programme the evening before his appeal bid.

Lisa Gribbin, 38, who was Claire’s best friend at Dartford Grammar School and also a trial prosecution witness, said: “I didn’t want to watch it at first but it was fantastic.

“It was quite weird sitting outside the box, so to speak, and watching what was going on. A couple of parts were very difficult to view but it was so good – by the end I was in tears. I thoroughly recommend people watch it.”

Last year, Sara and Blue received the Women in Film and TV News and Factual Award. Their previous work has included The Unspeakable Crime: Rape and The Bomb Squad, both screened on BBC One.

Of the forthcoming programme, director of public prosecutions and head of the CPS, Alison Saunders, said: "I was delighted that CPS colleagues from across the country were able to join me in taking part in this ground-breaking documentary which has shed light, for the very first time, on the way in which we make decisions and build cases to secure convictions.

"The three films follow some really moving, and legally challenging, cases which I hope will further the public's understanding of the important work CPS lawyers do day in, day out."