DETECTIVES are appearing on TV's Crimewatch tonight in the hope of catching an obsessive Bolton stalker who has tormented famous women for 20 years.

The man, who uses airmail envelopes normally bearing a Bolton postmark, has bombarded celebrities including ex-Avengers stars Joanna Lumley and Diana Rigg and Radio 4's Sue McGregor with explicit letters describing his sexual fantasies.

Forensic handwriting experts have linked thousands of items of obscene mail to the same man, whose other targets include Nyree Dawn Porter, who played Irene in the Forsyte Saga, and Dee Dee Wilde, founder of dance troupe Pan's People.

Dame Diana Rigg said: "He obviously has a thing about middle-aged actresses. We are all the same generation, give or take a few years."

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Detectives are hoping someone in Bolton will recognise the handwriting on envelopes addressed by the pervert and shown on the BBC1 show tonight.

They are also asking Bolton shopkeepers or post office workers if they have noticed anyone buying airmail envelopes in large quantities.

Investigations into the stalker have been re-opened after a letter was sent to an actress starring in a West End show at Christmas.

Joan Bakewell said the man's actions had caused considerable distress to the victims. She said: "This just has to come to an end. We've been harassed and patient long enough." One theory is that the man may have links with the theatre.

Det Sgt Shirley McGone, who is dealing with the case, said: "This is a very unusual case which has been going on for a number of years. The man's handwriting is very distinctive and should be easy to identify."

Only the envelopes can be shown on tonight's programme because of the pornographic nature of the actual letters.

Anyone with information about the stalker is asked to contact the Metropolitan police's incident room on 020 73218878 or Crimestoppers on free phone 0800 555111. Crimewatch is on BBC1 at 9.05pm.