Although Sussex Police said last night that they were keeping an open mind over her whereabouts, they added that a charge of wasting police time would be brought if she were found to have staged a vanishing act.
Kelvin MacKenzie, the editor of the Sun, was yesterday interviewed by police searching for Ms St Clair over a letter the newspaper received from her on 2 January. In it she offered to expose the 'perverted, corrupt and crooked' lives of 252 MPs who she claimed were her clients.
She wanted to meet a Sun reporter at a hotel in Brighton to discuss the allegations, but the newspaper declined, saying: 'We felt she was trying to lure us into a stunt designed to damage the paper and tabloids in general.'
Media excitement grew briefly yesterday when it was revealed that a piece of House of Commons notepaper had been discovered in a coat left in the Jaguar hired by Ms St Clair, and abandoned at Beachy Head, near Eastbourne in East Sussex, last Saturday.
But Detective Superintendent Mike Bennison said that only the top one-third of the paper was recovered. That included her address and the word 'Dear . . . ' but the rest had been torn off.
A reference at the top of the page is understood to have identified the MP who sent it. He will be interviewed, but Det Supt Bennison said that the letter was expected to be a routine piece of correspondence.
As 'Miss Whiplash', Ms St Clair founded the Corrective Party to campaign for prostitutes' rights, and was in regular correspondence with MPs.
Detectives have four theories as to what happened to Ms St Clair: that she was murdered; that she committed suicide; that the disappearance is a publicity stunt; or that she vanished deliberately to avoid paying her tax bill. The last theory is the one being most seriously considered. Detectives have found a witness who saw her leaving home with a large suitcase the night before she vanished.
'We have not found that suitcase,' Det Supt Bennison said. 'It is not in her hotel room; it was not in the Jaguar that was abandoned; and three witnesses who identified her walking away from Beachy Head (on Saturday) said that she wasn't carrying it then, so where is it? The disappearing act seems a fairly good theory with an elaborate trail of intrigue and an added bit of spice.'
Ms St Clair was due to meet a local newspaper journalist to reveal her 'dirt file' on more than 200 MPs and judges before she disappeared.
'When she spoke to the journalist she said: 'Someone is following me, I can't see you now', ' Det Supt Bennison said. 'It seems she has gone to the brink with her threats to expose but then couldn't come up with the goods. From what we have seized (from her home in south London) there is nothing to indicate that a 'dirt file' exists. There is nothing to suggest that anyone of any prominence has partaken of any sexual activity with her.'