Most historians believe the famous anti-monarchist died of malaria.

But American scholar Howard McCains believes letters dating from 1658 and the pattern of Cromwell's illness point to poisoning.

Cromwell- key dates

1599, born 25 April 1628 - Joined Parliament, representing Huntingdonshire 1642 - Civil war breaks out 1645 - Cromwell's New Model Army destroys King Charles I forces at the Battle of Naseby 1649 - King Charles I beheaded 1649 - Commonwealth established 1653 - Parliament disbanded. Cromwell appointed Lord Protector 1658 - dies, 3 September

Cromwell, one of English history's most controversial and enigmatic figures, most famously led a revolt and deposed the monarchy.

He died on 3 September 1658 after being cared for by a Dr Bates, who Mr McCains says was part of a royalist plot to kill Cromwell.

Mr McCains told the BBC's Radio 4 Today programme: "The pattern of symptoms, beginning on the first evidence of poor health on 31 July and carrying through to 3 September, lead me to believe he was poisoned."

Mr McCains said there was correspondence from Thomas Wood at the time which revealed a confession from Dr Bates.

'Oxford gossipmonger'

However, 17th Century expert and Cambridge University Professor John Morrill questioned the new theory.

He said: "Mr McCains has done tremendous work, but I don't think the evidence adds up to what he understands it to find."

Prof Morrill said Cromwell had been in poor health for a couple of years before his death. He had missed a number of council meetings.

And Mr Wood, who revealed the so-called confession, was no more than an Oxford gossipmonger, Prof Morrill added.

"I think Cromwell died from medical complications and from a broken heart because his favourite daughter had just died," said Prof Morrill.

Bloody revolt

With his anti-monarchist revolt, Cromwell inspired the beginnings of a more democratic society, but his methods were often brutal and bloody.

Charles I: Executed in 1649

This has led commentators and historians to interpret his character and motives in radically different ways.

In 1645, Cromwell's New Model Army famously destroyed the king's forces at the Battle of Naseby.

Many hold Cromwell responsible for the execution of Charles I in January 1649, although there were 59 signatories to the death warrant.

Despite this opposition, Cromwell established his status and authority. Supported by the army, he was appointed Lord Protector in 1653.

When he died in 1658, England was prosperous and the seeds of a constitutional government had been sown. But he failed to establish a written constitution or leave a lasting system of government.

Consensus of opinion remains elusive, however. Nearly 400 years of debate have failed to settle the issue of his reputation.