LONDON, ENGLAND - NOVEMBER 23: Boris Johnson attends the London Evening Standard Theatre Awards at the Royal Opera House on November 23, 2009 in London, England. (Photo by Ian Gavan/Getty Images)

John Hyde

BORIS Johnson is set to break his pre-election promise to keep the Tube running later at weekends.

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BORIS Johnson is set to break his pre-election promise to keep the Tube running later at weekends.

The one-hour extension was a key pledge of Mr Johnson's mayoral campaign in 2008.

Transport for London says the plan is impossible to implement at the moment and describes it as merely an "aspiration".

The mayor has been criticised by his opponents, who argue that running trains later on Fridays and Saturdays would improve public safety and boost London's night economy.

"The mayor was elected promising to extend the opening hours of the whole Tube network," said Lib-Dem London transport spokeswoman Caroline Pidgeon.

"It is now time he honoured his pledge, starting with the Jubilee line, where the upgrade works are completed this year.

"Everyone who uses this line has already faced months of inconvenience and many traders that rely on weekend and evening business have been hit for six."

However, ongoing problems with upgrading the Underground to expand capacity stand in the way of Mr Johnson keeping his pledge.

He was grilled at mayor's question time last week on the issue, when he said: "We are going through a laborious process of upgrading the Tubes and it is not possible to do it at the moment."

A TfL spokeswoman clarified the mayor's comments, saying: "Given the context of the many timetable changes required to facilitate the line upgrade and the volume of weekend work under way, introducing a later running would be extremely difficult and complex at present.