Ian Brady's mental health advocate Jackie Powell Photo: PA

Moors murderer Ian Brady's legal advocate has been arrested for allegedly failing to reveal information about where one of his victims is buried, sources have confirmed.

Jackie Powell was detained in south Wales on suspicion of preventing the burial of a body without lawful exercise.

Ms Powell, who was appointed Brady's legal advocate under the Mental Health Act in 1999, has told a Channel 4 documentary that Brady gave her a sealed envelope to pass on to Winnie Johnson, the mother of his 12-year-old victim Keith Bennett, in the event of Brady's death.

Greater Manchester Police said in a statement that Brady had finally disclosed details of the location of the body to one of his "long-term visitors" at Ashworth high-security hospital, where he is being detained.

The force did not name the suspect but said the 49-year-old woman had been arrested in south Wales and remained in police custody for questioning.

The makers of the documentary said they had not seen the envelope.

Documentaries commissioning editor and editor for Cutting Edge Emma Cooper said: "No-one can verify the contents of the envelope and therefore what information it does or does not contain but given the enormity of the implications as suggested by Brady's letter, we felt we had a responsibility to inform the family via their family liaison officer.

"Winnie's poor health was foremost in our decision-making and this is why we have decided to bring forward the broadcast of the film and make the existence of the envelope public."

The film, Ian Brady: Endgames of a Psychopath, will be broadcast on Monday night at 9pm

In an interview she for the programme, Ms Powell said she received a letter of instruction and a sealed envelope from Brady via his solicitors.

She explained that the letter of instruction states the additional envelope contains three letters - one apparently addressed to Winnie Johnson.

Ms Powell, who has professional obligations in her capacity as executor and her role as Brady's Mental Health Advocate, told the documentary she has not opened the sealed envelope, she cannot be certain of its contents.

Sources said detectives were examining documents seized from Ms Powell's home but as of yet there was no evidence to suggest Brady has disclosed the location of Keith Bennett's body.

Ms Powell has since been released on bail pending further inquiries, the source added.

Mr Ainley, senior partner at Oldham-based North Halliwell Ainley, said of Mrs Johnson: "She has remained steadfast in her resolve that Ian Brady has held the key to this tragedy.

"Over the years she has appealed directly to Brady and through me for him to reveal where Keith's body was hidden so she could at least give her son a decent burial before she died.

"Now, she herself is ill and I am deeply concerned that the knowledge of this letter and its alleged contents may have come too late to bring her any solace.

"It would be the last tragic act of a truly harrowing story."