He was one of Britain's greatest botanists - a man of humble origins who rose to become a distinguished professor and a Fellow of the Royal Society.

But it now transpires that John Heslop Harrison was more than just a working-class hero who made good. He was a cheat and a scientific fraud who provided botany with its equivalent of the Piltdown hoax - the skull supposed to be the 'missing link' between men and apes, later proved to be a fraud.

In the post-war years, Harrison took rare grasses, sedges and rushes and planted them on Rum in the Inner Hebrides. Then he returned to the mainland, mounted expeditions back to the island and there, with great incredulity, 'discovered' them.

When it came to planting evidence, no one could touch Heslop Harrison. But why? Simple: Harrison wanted to provide 'conclusive proof' that his theory - that some plants had survived the last Ice Age, even though they were buried under glaciers for 10,000 years - was correct.

Harrison was wrong, and the startling nature of his evidence eventually led him to be suspected of fraud. A report by a fellow academic damned him for his 'complete fabrication'.

But the document was never published and languished in the library of King's College, Cambridge, until it was discovered recently by film-maker Karl Sabbagh, whose book on the incident, A Rum Affair, will be published by Allen Lane next month.

'Heslop Harrison was a dogmatic bully who indulged in chicanery,' said Sabbagh. 'The trouble is that nothing has changed since his day. We still have fraud, yet we still hate making a fuss about it - even to the extent of suppressing evidence about major scientific cheats. That is the real lesson to be gained from this story.'

Harrison was the son of an iron-worker but rose to become a professor of botany at Newcastle University and a distinguished botanical researcher, much of his reputation resting on his work on Rum. Harrison was given unrestricted access to this remote island by its owners, the Bullough family, and used this scientific monopoly to make several sensational 'discoveries' of plants that no researcher had previously expected to find there.

But Harrison was an awkward, difficult man who made enemies easily (wearing a Hitler moustache didn't help, says Sabbagh) and, after years of producing endless botanical rabbits from a hat, he aroused suspicions. Academics noted that Harrison was always apart from his students when he made a discovery and would often dig up specimens before their location could be revealed.

Eventually, Cambridge classics tutor and amateur botanist John Raven asked Trinity College for a £50 grant so he could make a clandestine trip to Rum. He arrived in 1948 and, while being devoured by midges, began to examine Harrison's colonies of plants.

To his consternation, Raven found that some of Harrison's rare plants had clearly been dug in with a trowel while others were uncontaminated by weeds, unlike other plant colonies on the island, showing they were newcomers.

Raven wrote a report which concluded that Harrison was 'deliberately indulging in the most culpable dishonesty in order to secure for himself an immediate reputation and an immortal place in the annals of British botany'.

But after Raven's report was presented to Trinity and discussed by several scientists, it was buried in the library. And there it would have remained had it not been mentioned, in passing, in Raven's obituary in 1980. Sabbagh decided to investigate, and eventually discovered the report and emphatic evidence of Harrison's 'chicanery of a high order'.

But why was Raven's report suppressed? 'I think the scientific establishment closed ranks because they felt that Harrison's frauds had not done a great deal of damage. He had not managed to mislead science for very long.'

The fact that Harrison's son, and later his grandson, became distinguished botanists was also relevant, added Sabbagh.