A copy of a previously unknown letter by Robert the Bruce imploring King Edward II to cease his persecution of the Scots in the build-up to Bannockburn has been discovered.

The letter, sent in 1310, shows Robert asserting his god-given authority as king of Scots and addressing Edward as his equal.

The letter has presented historians with fresh information about a pivotal time in the wars of Scottish independence as Scotland prepares to commemorate the 700th anniversary of the decisive Battle of Bannockburn next year.

Discovered by chance by a Glasgow historian, the document reveals how Robert made an appeal for peace when faced with an English army marching into Scotland, on the understanding that Edward would recognise Scottish independence.

The document, thought to have been transcribed around the turn of the 16th century from the original letter, provides new insight into a time when Robert and Edward were experiencing different fortunes.

Edward was growing increasingly unpopular with the nobles in his own court, according to historians, while Robert was slowly reclaiming power north of the Border by winning the hearts and minds of the Scottish people.

Dauvit Broun, professor of Scottish history at the University of Glasgow, who made the discovery, said: "The letter reveals a couple of things: first, Bruce's tone is extremely conciliatory. He seems to be offering to do anything to establish peace.

"However, he is nonetheless plainly addressing Edward as one king to another. There is no doubt that the bottom line here is that Edward should recognise Robert as king of the Scots and the Scots as separate from the English."

The letter, translated from its original Latin, states: "To the most serene prince the lord Edward by God's grace illustrious king of England, (from) Robert by the same grace king of Scots."

"Our humbleness has led us... to beseech your highness more devoutly so that you would take pains to cease from our persecution and the disturbance of the people of our kingdom in order that devastation and the spilling of a neighbour's blood may henceforth stop."