A letter linking Mary Queen of Scots, her husband Lord Darnley, and his future murderer, Lord Bothwell, has surfaced after more than 430 years.

In the letter, written in August 1566, the Queen commands Bothwell - whom she married the following year just three months after becoming a widow - to capture a band of rebels terrorising Sir William Livingston, one of her most loyal supporters.

Written in old Scots, the document symbolises the intrigue and political tensions surrounding one of the most colourful and turbulent times in British history.

It was issued jointly in the names of Henry and Mary - ''be ye Grace of God King and quene of Scottis'' - but is signed only by the Queen, indicating the ambiguous relationship of the royal couple just two months after the birth of their only son, the future James I of England. Now the 30-line document, written at the time of the final estrangement of the couple which led to Darnley's murder six months later in Bothwell's gunpowder plot in Kirk o'Field, Edinburgh, is set to fetch up to #30,000 at Christie's in London on June 3.

Christie's specialist Thomas Venning said yesterday: ''It is a rare and evocative letter capturing the tensions of the time. At first, Mary was infatuated with Darnley and on the eve of her wedding in 1565 proclaimed he was to be called King of Scotland.

''But he was deeply unpopular and his part in the murder of David Rizzio (the Queen's private secretary) confirmed her own profound distrust and dislike of him.''

By the time the letter was written, the much-criticised marriage was effectively over and, in 1587, three months after Darnley's death, Mary married the Protestant Lord Bothwell at Holyrood Palace, apparently after he abducted and raped her. Three months later, she renounced the Scottish throne.

The letter, from a private collection, was written by Mary to Lord Bothwell in his capacity as sheriff of Berwick, Haddington, and Edinburgh and baillie of Lauderdale.

The Queen urges him to seek out and apprehend a band of eight named rebels who had been living on the land of Sir William Livingston of Kilsyth and were responsible for burning his corn and pursuing his wife and servants with the intention of killing them. If no appropriate punishment was meted out it would encourage others to commit similar offences - ''And gif condigne punissment be not execute there It will give boldness & occasion to sik wicket personis to comit ye like contempt and tressonable deid To ye greit inquietatioun and hurt of our commone weill & obedient subjects.''

The letter commands Bothwell in the name of the king and queen to capture the rebels - and others like them who might be caught.

q At the same sale, a 1558 land document bearing a fine and rare signature by Elizabeth should fetch up to #15,000.