The best queen England never had

She is the murderous monarch, beautiful but bad, who killed her husband, Lord Darnley, in order to wed the Earl of Bothwell.

Or she was a peacemaker, anxious to keep on good terms with England and not let her Catholic faith cause trouble with a recently Protestant Scotland, ruling as a fair monarch - an aim ultimately doomed to failure by circumstances beyond her control.

But there is a another picture: a successful monarch - "the best England never had".

Research by Professor John Guy, of St Andrews University, has painted a new picture of Mary as a queen who, had she lived, would have eclipsed her "cousin", Elizabeth I, as monarch of the United Kingdom, a throne she bequeathed to her son, James Vl.

Prof Guy claims Mary was innocent of Darnley’s murder and was the victim of a conspiracy orchestrated by William Cecil, the most powerful man in England.

For 40 years, Cecil was the architect of Elizabeth’s policies, and in a new book, Prof Guy says his machinations brilliantly focused the disparate agendas of nobles around the Scottish throne.

He said: "It’s time for Scotland to rediscover a great queen, charismatic, successful, a modern woman who secured the succession and was viewed with favour by England.

"She was Catholic, but was tolerant of the Reformed faith."

But growing stature and Catholicism were anathema to Cecil, who wanted a unified Protestant Britain.

Driven by that desire, and unknown to Elizabeth, he initiated his plot. His vision was realised in 1587, at Fortheringay, where Mary was beheaded after Cecil persuaded Elizabeth she had to die.

Mary’s execution and the death of the childless Elizabeth allowed Mary’s son, James, a Protestant, to become the first monarch of a United Kingdom.

Prof Guy added: "Cecil then re-wrote history, falsifying documents, to present us with an enemy’s version of Mary. His plot lasted years and took advantage of the disgruntled Scottish noble, James Douglas, the Earl of Morton."

It was Morton who masterminded the Darnley murder at Kirk O’Field in Edinburgh in 1567, says Prof Guy, contrary to the belief that it was Bothwell.

Years before, Morton, with Bothwell and other nobles and backed by Darnley, had murdered Mary’s confidant, David Rizzio. As a result, Morton was exiled to England, where he met Cecil and told him he "would do anything" for him.

Cecil engineered Morton’s return to Scotland during the baptism of the infant James Vl.

Prof Guy added: "I went through everything and it dispels the belief that Mary and Elizabeth were enemies."

Which is why, he says, when Mary was fleeing Scotland, she threw herself on the mercy of her "sister".

Prof Guy said: "Cecil needed Mary destroyed. Darnley’s murder was part of it."

Darnley’s killing came, he added, as Mary’s ambassador was leaving for London to sign a dynastic accord that promised Mary and her heirs the throne of England.

Morton led Darnley’s killers, but the queen’s husband, who was lying ill in Kirk O’Field, Edinburgh, survived an explosion.

He was pursued by Bothwell, Sir James Balfour and the Douglasses under Sir William Ker, who strangled him.

Later, Mary took action against the plotters but unwittingly named Bothwell as Queen’s protector.

Bothwell became too powerful and when he threatened to eclipse Morton, his erstwhile ally tried to destroy him.

In retaliation, Bothwell abducted and married Mary.

The queen’s popularity was destroyed. War followed and Mary abdicated in favour of James. But while she lived and Elizabeth was childless, the Catholic queen was a danger.

Elizabeth would never have dreamt of killing a sister queen, but she was worn down by Cecil’s intrigues into believing the Spanish were going to rescue Mary.

She signed her death warrant, and it was only then the Spanish acted, sending the ill-fated Armada to avenge her.

Prof Guy said: "Every monarch since comes from Mary’s bloodline. The truth is she would have been a great queen.

Final proof of Cecil’s plot emerged with Morton’s last words. Eventually he was tried for Darnley’s murder and revealed it was orchestrated by a "great person" in England.

Until his death in 1589, Cecil was known as the "great personage".

My Heart is my Own, the life of Mary Queen of Scots, is published on 15 January at 20.