Lazard has earned $25bn (€20bn) in league table credits for defending UK airports operator BAA, despite having no official mandate to work on the deal and not receiving a fee.

Investment banking data providers Dealogic and Thomson granted the investment bank more than $25bn worth of credits because Marcus Agius, non-executive chairman of BAA, sought advice on bid tactics from colleagues at Lazard's operation in London, where he is also chairman.