Sanofi is seeking an injunction to prevent Sandoz bringing the copy to market. Such legal actions are common against generic drug manufacturers but it is rare for two major companies to come to blows. Sanofi, which is based in France, is already fighting attempts by two other generic makers, Amphastar, which is based in the US, and Teva of Israel, to copy Lovenox.

The drug, which acts as an anticoagulant, had sales of $1.2 billion (£630 million) in the first half, just under 10 per cent of Sanofi’s total sales.

Another of Sanofi’s blockbuster drugs, Plavix, also an anti-clotting treatment, is being challenged by generic companies. The company reached a settlement with Apotex this week that allows a generic version of Plavix to be made from 2011. Plavix is the world’s second bestselling drug with earnings of $6 billion last year.

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