The group's finance director said that there was no evidence of fraud or deliberate manipulation of the numbers and the errors were simply mistakes.

There has been concern for years over the industry's deliberate manipulation of adverse data and there has been frustration at the failure of medical regulators to address the issue.

But all were guilty, at times, of underhand behaviour — false accounting or creative repainting or the deliberate manipulation of prices.

If this was a deliberate manipulation effort it failed, however, as the price of the contract rebounded rapidly to its previous level.