Others point to possible cross-subsidy from the monopolised settlement arm to the trading platform.

ECONOMIST: Securities exchanges

Over the past few months it has looked at the cross-subsidy between retail and investment banking in universal banks.

ECONOMIST: Two views on new banking rules are better than one

"We don't think it would be financially viable to build another runway at Stansted without cross-subsidy, " a BA spokesman told Reuters.

CNN: UK's Stansted expansion challenged

With wholesale markets immature or non-existent in much of Europe, pricing remains open to manipulation and cross-subsidy to keep newcomers out.

ECONOMIST: Europe should speed up the opening of its energy markets

For their part, most American and British firms have bowed to the stockmarket's view that mixing the two businesses results in an inefficient cross-subsidy.

ECONOMIST: The chemicals business

They have raised the possibility of less cross-subsidy within UK pension schemes, and "a regulatory black hole" where Scottish pensions are currently regulated by UK-wide bodes.

BBC: sterling

On September 28th, the Civil Aviation Authority, the airports' regulator, is to recommend a new method of price regulation, which will end the cross-subsidy of airport charges.

ECONOMIST: Airports

So the cross-subsidy system won't work.

BBC: Free nursery scheme '55,000 places short'