London Mayor Ken Livingstone and his transport commissioner Bob Kiley are to launch a new High Court challenge against the Government's part privatisation of the Tube.

The Government recently announced it was pressing ahead with the �16bn investment plan over the next 15 years despite opposition from passenger groups and the unions.

With the agreement of all sides, Mr Justice Scott Baker has now ordered an urgent hearing to take place over four days beginning on 23 July.

Lord Lester of Herne Hill QC, appearing for the Mayor, applied for a quick hearing.

'Funding gap'

He said: "Everyone agrees that it is in the compelling public interest, as well as in the interests of the commercial parties, that this should be expedited."

The case marks the Mayor's second legal challenge to the controversial Public Private Partnership (PPP) proposals.

It is among the first serious actions to be faced by new Transport Secretary Alistair Darling.

Lawyers for Mr Livingstone are arguing that the arrangements for PPP violate European funding and procurement rules.

It is also argued they will create a "funding gap" of �1.5bn, which would require a 25% fares.

Under PPP the Tube will be split into four main sections.

Delays

Two private consortia, Metronet and Tube Lines Group, will control the lines and infrastructure under 30-year contracts.

The running of trains and operation of signals will remain under public control.

Recent reports suggested the Government wanted the first substantial contracts with the private-sector consortia signed by the end of this month, but that is now expected to be delayed.

These contracts involve the Tube Lines consortium, which will take over the Jubilee, Northern and Piccadilly lines.

A spokesperson for the Mayor said: "He will be highly delighted and very pleased that we have the go-ahead."

A London Underground spokesman said: "We are sorry to be here in the first place, but we are confident that we have a good plan and that we have a good case."