As historical whodunits go, it is one of the most compelling of them all. Alexander the Great, overlord of an empire stretching from Greece to India, was suddenly and inexplicably cut off in his prime at just 32.

Over the centuries, suspicion has fallen on any number of potential poisoners, from Alexander's own wife and illegitimate half-brother to his generals and even the royal cup-bearer. But now a British historian believes he has finally solved the mystery: the killer of the greatest warlord in human history was nothing more than a humble mosquito.

Andrew Chugg, a respected authority on Alexander and author of a number of books on the subject, claims he has unearthed new evidence to suggest that the Macedonian conqueror died of malaria, contracted two weeks before his death while sailing in the marshes outside Babylon.

Mr Chugg bases his argument on the Ephemeredes, or Journal of Alexander, an ancient diary kept by one of the king's aides during his reign in the late 4th century BC. Critics have long questioned the authenticity of this source, claiming it was an elaborate later fabrication, but Mr Chugg believes he has now identified a number of "very sharp coincidences" that prove its veracity.

The author, he says, was Diognetus of Erythrae, a surveyor in Alexander's entourage, responsible for mapping out territories as they were conquered. The journal gives a detailed account of Alexander's last days, describing symptoms highly consistent with malaria.

"If you accept that the Ephemeredes is an accurate description of how Alexander died over a period of two weeks, then it is very difficult to believe the poisoning stories," said Mr Chugg, who will put his case in a forthcoming edition of the academic journal Ancient History Bulletin.

"It is known that the incubation period for malaria is between nine and 14 days, and about this time before he fell ill, we know that Alexander was sailing around the marshes outside Babylon inspecting flood defences. This is almost certainly where he was infected. Throughout history, and even to the present day, people have been contracting malaria in that particular area."

Mr Chugg claims he can also prove the authenticity of a second contemporary source: a commentary on Alexander's journal by another man who served under him, Ephippus of Olynthus, which further strengthens his case.

"It is difficult to imagine how anyone could have been fooled into compiling the commentary at such an early date if the journal itself had been fabricated," said Mr Chugg. "Ephippus of Olynthus, who had served Alexander in Egypt, was the author of an indisputably genuine book about Alexander's death."

Alexander, who died in Babylon in the summer of 323 BC, was last year the subject of a Hollywood "sword and sandals" epic, starring Colin Farrell and Angelina Jolie, as well as two biographies supporting the poisoning theory.