Clive Fairweather's contract expires in October

He said that the chief inspector's contract had already been extended twice and that his appointment was simply "coming to a natural end".

However, Scottish National Party justice spokeswoman Roseanna Cunningham said Mr Fairweather had paid the price for doing his job too well.

And she also compared the move to the departure of former Parliamentary Standards Commissioner Elizabeth Filkin, who left her post amid claims that she had been pushed out.

Mr Fairweather took up the post in October 1994 after serving 32 years in the Army.

Adverts for the post, which carries a salary of �50,000, appeared in newspapers at the weekend.

The appointment will initially be for a four-year period.

Appointment extended

Mr Fairweather has been an outspoken critic of failings in the prison service on issues such as slopping out, overcrowding and the number of women prisoners in jail.

However, the Scottish Executive spokesman stressed: "Mr Fairweather has already had his appointment extended twice.

"On the last occasion it was made clear this would be the final extension.

"This is simply a case of the appointment coming to a natural end."