A toxicologist from Leeds University has played down claims from Labour's shadow environment secretary Mary Creagh that the drug phenylbutazone causes cancer in humans.
Professor Alistair Hay said there is "no convincing evidence of carcinogenicity".
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At least 2,400 cancer sufferers die needlessly every year because their GP does not refer them to a specialist quickly enough.
It comes after shadow chancellor John McDonnell made a surprise U-turn on the Tory budget vote which is due later today.
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