Detective Chief Inspector Ewen West said it was “crucial” the writer got back in touch and assured her that any information will be treated in the strictest of confidence and guaranteed her anonymity will be protected.

“Through detailed investigation and appeals in the media, we have gathered significant new information,” said DCI West.

“Detectives in the team are now in the process of sifting through that information and identifying further people we need to see.

“We would particularly like to hear from a lady in Dundee who wrote us an anonymous letter.

“She may have very important information for us and it’s crucial that we talk to her.

“I can reassure her that her anonymity will be protected at all times and we would only ever talk to her at a time and place that is convenient to her.

“If that lady would prefer to talk to a female police officer I would ask her to contact Detective Sergeant Carol Dunn, who is a member of the inquiry team.

“She can be contacted in the incident room on 01382 591909.”

DCI West appealed two weeks ago to people with links to Aberdeenshire and prostitution.

He said the use of ground-breaking forensic techniques, not available at the time of the murder, has thrown up new information.

Behavioural and geographical specialists have been drafted in and they have identified priority lines of inquiry for the investigation team.

Carol’s body was found by a dog walker at Templeton Woods on the outskirts of Dundee on March 22, 1979.

She had left her home in Hill Street on the night of March 20 and was known to have been in the Castle Street and Exchange Street areas of the city centre.

She was last seen wearing a white polo neck jumper, brown mid-length striped skirt, black velvet jacket and black shoes.

On April 1, 1979, some of Carol’s belongings were found on the banks of the River Don near the A96 between Hatton of Fintry and Kintore, Aberdeenshire.

With her being known to have visited the city’s then red light district on the night she disappeared, police are urging people who may have been associated with or used the services of prostitutes during 1979 to come forward.

While understanding many of these people may now have a settled family life, DCI West assured information would be treated in confidence.

Police also believe there may be a connection with the Kintore area of Aberdeenshire where her bag was found.

Anyone with information can call police on 01382 591909 or 01382 223200, or email operation.trinity@tayside.pnn.police.uk or send a text message to 07980 543641.