Rob Roy's life was celebrated in a film starring Liam Neeson

He spent the last 20 years of his life, until his death in 1734, dodging arrest and stealing cattle from the duke's lands, which he then sold in public, after vowing to "do all justice to my creditors".

The text of the letter, written in 1724 when Rob Roy was living as an outlaw, is signed "Ro: Roy".

It calls on Montrose's factor Buchanan to arrange a meeting to pay a sum of money.

Historical researcher Dr Sebastian Pryke said: "The Rob Roy letters that survive are generally signed Rob Campbell, so consequently this letter signed Rob Roy has a huge emotive value.

'Enormous interest'

"It's very possibly the only surviving document of its kind. A lot of the documentation may have been destroyed because he was a Jacobite, and it is also very likely that he would have avoided putting pen to paper as much as he could.

"We have estimated that it will sell for �8,000 to �12,000, but I think there will be an enormous amount of interest and I wouldn't be surprised if it sold for more than that."

Rob Roy's life was romanticised in Sir Walter Scott's Waverley novels and more recently by the Hollywood film starring Liam Neeson.

Other items due to be auctioned in the sale include an Act of Union manuscript from 1707, valued at �4,000 to �6,000, and a letter from Samuel Pepys to William Penn.

The auction will be held next month.