A LETTER posted to Southampton could be the key to solving the mystery behind one of television’s most popular dramas.

It features in Broadchurch, ITV’s tale of murder set in Dorset starring former Doctor Who David Tennant and Olivia Colman.

Tennant’s character, Detective Inspector Alec Hardy discovered a letter, posted to a PO Box address in Southampton, hidden in the wardrobe of Claire Ashworth, a woman he is keeping safe from the prime suspect in the fictitious Sandbrook child murder.

Det Insp Hardy led the investigation but failed to prove that Claire’s husband, Lee, killed a 12-year-old girl and her older cousin, before he moved to Broadchurch – a fictional close-knit town based on West Bay in Dorset.

Mystery still surrounds the origin of the letter, which is set to be an integral part of the plot to finding out what happened in the Sandbrook case.

Daily Echo:

It is one of the main storylines in the second series of the award-winning drama, in which Coleman plays Det Insp Hardy’s colleague, Ellie Miller.

Det Insp Hardy opened the letter to find a dried bluebell pressed inside a folded piece of white paper.

The flowers feature in some of the flashbacks to the Sandbrook case.

In the latest episode, on Monday night, DI Hardy questions Claire about the letter, but she insists she does not know who sent it.

She told him it was sent to an address she used to use for hair supplies, when she worked as a hairdresser.

The storyline runs alongside the main plot of Danny Latimer’s murder trial, |the investigation for which was the basis of the first series, which won the show and its performers massive acclaim from both the critics and the public.

This series follows the court case of Joe Miller, Ellie’s husband, who shocked everyone by pleading not guilty to the young boy’s murder, despite his confession.

The new series has come under fire from critics and lost 1.6 million viewers between the first and second episodes.

Monday night was the lowest ever audience for Broadchurch at 5.7 million.

It even fell behind BBC1 rival Silent Witness which pulled in 5.9 million viewers.

The first series debuted with an audience of 6.8 million in March 2013, with 8.7 million watching its finale the following month.