Constable Sarah Locker, 31, whose allegations of racial and sexual discrimination are being heard by a separate industrial tribunal, claims that she was repeatedly refused promotion to the CID and put through gruelling interviews while pregnant by officers investigating her complaints.
After beginning grievance procedures, Mrs Locker, who is based at Leman Street police station in Whitechapel, east London, received an anonymous telephone call warning her that she could be in danger.
Her case is being backed by the Commission for Racial Equality and the Equal Opportunities Commission.
Vivienne Gaye, her counsel, told the appeals tribunal that the letter, which Mrs Locker says she was given by Detective Chief Superintendent Roger Stoodley in front of her colleagues, was an example of the harassment to which her client had been subjected.
The tribunal is hearing an appeal by the police against the disclosure to Mrs Locker's lawyers of documents concerning internal grievance procedures. Among them are papers relating to an investigation into the provenance of the letter.
The letter, from which Ms Gaye quoted, takes the form of a mock application for specialised training supposedly written by Mrs Locker to Wyn Jones, Assistant Commissioner. The police claim that she was given it by a junior officer as a joke.
The letter, which was released by Mrs Locker's solicitors, quotes her as saying: 'A few of them courses is right up the street of a third world effnic girl like me . . .
'I could really go for that Rape and Serious Sexual Offences bit - if they got some nice greasy Mediterrean (sic) boys there, wiv all the right bits cut off their fings - the practice might come in handy.'
In relation to a course on police and ethnic minorities, Mrs Locker supposedly says: 'Me being a turk an all that (well I was born in Stepney but everybodys in my family is from one of them sweaty little places) I fink I'd teach them dick heads a fing or two.'
The letter makes offensive reference to Mrs Locker's internal complaints and ends: 'And wots more me mum is into magic . . . and one word from me and the curses will be flying about faster than a pork chop in a Mosque.'
The decision of the appeals tribunal is expected within the next fortnight.
Mrs Locker's solicitor, Jane Deighton, said after the hearing: 'This is an appalling, horrific letter of which the force should be deeply ashamed.
'If her claim is proven, it casts a whole new light on those who control promotion procedures.'