The dominance of the Big Four accounting firms is partly self-perpetuating. The world's largest companies employ the world's largest auditors; ergo, if I want my company to become one of the world's largest, I ought to employ one, too.

Such reasoning may be wrong-headed, but in a free market, it would be difficult, undesirable and paradoxical to impose "choice" on audit firms' corporate clients. The problem, according to some of those who have now responded to the Financial Reporting Council's inquiry into the UK audit market, is that intermediaries such as banks are further skewing the playing field by urging their clients to "move up" to the Big Four. Clauses in loan agreements can also limit the choice of auditor. According to BDO Stoy Hayward, one of the next tier of auditors, such obligations are "a major barrier to entry" for firms trying to win business from big blue-chip companies and are reinforcing "institutionalised prejudice".