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Boris Johnson's calling on the government to step in and save Londoners millions of pounds as the row about upgrading the tube becomes increasingly bitter.
The company responsible for improving the Jubilee, Northern and Piccadilly Lines wants nearly £6Bn to do the work over the next 7 and a half years.
London Underground says it can be done for nearer 4 billion, and the independent arbiter called in to sort it out has put the cost at £4.4Bn.
The mayor's now written to the Transport Secretary to ask the government to make up the shortfall or force Tube Lines' shareholders to give up their fees.