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Mayor Boris Johnson has backed skateboarders fighting to keep their Thames-side home in a shock intervention provoking fury from the Southbank Centre.

Speaking at a preliminary planning meeting, Mr Johnson said he supported the controversial plans to redevelop the centre with a revamped “Festival Wing” and new glass pavilion only if the existing skate park in the undercroft was retained.

He said he was “encouraged by many of the aspects” of the plans but said redevelopment should not be at the detriment of the skate park which should be kept where it was.

“The skate park is the epicentre of UK skateboarding and is part of the cultural fabric of London. This much-loved community space has been used by thousands of young people over the years. It attracts tourists from across the world and undoubtedly adds to the vibrancy of the area - it helps to make London the great city it is.”

The Southbank Centre has pledged £1million for an alternative skate site 120 metres away under Hungerford Bridge.

But the mayor’s intervention is a severe blow because the decision to be made by Lambeth Council will be eventually referred back to him.

A Southbank Centre spokesman said they wanted to know how Mr Johnson expected them to fund their work. New restaurants and shops planned for the undercroft would pay for the £120 million redevelopment and provide revenue at a time of government funding cuts.

“We are surprised by the mayor’s unexpected statement. We look forward to hearing how he intends to fill the financial gap that now stands between us and our ability to provide free art and culture to millions of Londoners.”

The board would have to consider the implications for the future of the project if the mayor failed to back it, the spokesman added.

A further blow has come from the 20th Century Society conservation group. Catherine Croft, its director, said: “We would never consider treating an iconic building of any previous century with the arrogant disdain of the current scheme for the Southbank Centre.

“The proposed massive extensions to the Southbank Centre will overwhelm one of the best groups of brutalist buildings in the UK, if not the world. Attempts to justify such desecration with spurious arguments about financial viability and wider public benefit are just ridiculous.”

Skateboarders: We're stoked, Boris is a real G

Skaters said they were "stoked" to get on the mayor's support in their fight to remain at the undercroft at the Southbank Centre.

One skater, Tommy Wright, said: “We’re stoked the Mayor is backing us, anyone down for preservation gets mad props. Boris is a real G.”

According to the Urban Dictionary, a ‘real G’ is “a robust individual who did not change course under rough circumstances”.

Campaigner Louis Woodhead, 18, said: “I’m very happy that he has come out to support us as this is the least popular planning application of all time. It’s a big step forward for the campaign, especially as he has the power to veto any decision.”

Jason Caines, 24, a freelance writer, said: “We are pretty proud of Boris to stand up and make a statement. It’s amazing. He believes that BMX bikes and street culture are worth preserving.”

Paul Richards, 36, creative director of arts organisation UpRise, who is involved in the Long Live Southbank campaign, said: “It’s great that Boris recognises the importance of the cultural heritage that exists in this space.” While the skaters continue their campaign to keep their skatepark, another group has entered the debate to mock the plans.

Conservationists from the Twentieth Century Society have released graphics showing how the proposed glass pavilion, by architects Feilden Clegg Bradley, could look if placed on top of London’s other cultural landmarks.

Director Catherine Croft said: “We would never consider treating an iconic building of any previous century with the arrogant disdain of the current scheme for the Southbank Centre.”