Arsenic killed Australia's greatest racing horse Phar Lap 74 years ago, a new scientific study reveals, The Daily Telegraph reports.

The story suggests a lethal dose of the poison was given to the Melbourne Cup champion about 35 hours before he collapsed and died in California on April 5, 1932.

Previous theories speculated the champion died of a stomach condition but many suspected foul play by US gangsters trying to avoid losses from Phar Lap's continued success.

Phar Lap won 37 of 51 races in his four-year career including the 1930 Melbourne Cup.

A sample of the horse's preserved skin was sent to a US laboratory in June and analysed by a synchrotron - a light-emitting particle accelerator.

"The arsenic in the hair structure is consistent with ... a single large dose of arsenic between one to two days prior to death," the report states.

But Australian scientists who participated in the study told The Daily Telegraph arsenic associated with traces of lead were also found in the sample.

The lead arsenate may have been used to preserve the hide but produces a different chemical signature and distinct from ingested arsenic, possibly used to poison Phar Lap.

"You will never get a 100 per cent definite answer that your confident there is no doubt about it," Australian Synchrotron Research Program scientist Ivan Kempson told the Daily Telegraph.

"But we can't explain it by any other way than the scenario of poisoning."

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