As a result, private companies have been forced to borrow from underground finance companies at often usurious interest rates.

FORBES: Junk Bonds Come To China

Mr Livingstone's preferred alternative is a bond issue to finance much-needed investment in the Underground, which would remain wholly publicly owned.

ECONOMIST: The London Underground

Ajit Chambers is a former City of London banker who quit finance to set up the Old London Underground Company in 2009, which is leasing the subterranean land from the city.

BBC: Reawakening the Underground