News: the first image showing the "super-strength" copper-nickel skin that will clad Thomas Heatherwick's Garden Bridge has been unveiled, just 24 hours after the £175 million project was threatened by a legal challenge.

The proposed 367-metre London bridge will be coated in a corrosion-resistant copper-nickel alloy from its feet on the river bed up to the underside of the bridge deck. This should make the underside of the bridge maintenance-free for 120 years.

Around 240 tonnes of the material known as cupro-nickel, which is typically used for mechanical equipment and ship propellers, will be roll-bonded to the bridge's structural steel form to create an inseparable skin.

The Garden Bridge Trust – the non-profit organisation behind the project – has described the copper as "a fundamental part of designer Thomas Heatherwick's concept" for the pedestrian bridge, which will span the River Thames between the South Bank and Temple.

"The copper-nickel skin, is an integral part of the design and upkeep of the bridge and just as important is the stunning appearance that the warm colour of the material will create," said Mervyn Davies, chairman of the trust.

The material is paid for exclusively by a donation from multinational mining company Glencore, bringing the total funds pledged up to £120 million. The remaining £55 million still needs to be found.

The new image is revealed just 24 hours after reports of a legal challenge to the project from local resident Michael Ball, the former director of charity and community planning organisation Waterloo Community Development Group.

Ball's lawyers claim that the decision to grant planning permission for the bridge was "unlawful", stating that Lambeth Council – one of two local authorities responsible – has failed to consider the impact the structure will have on surrounding historic buildings.

"The impact of the Garden Bridge will be devastating," said Ball in a statement. "The best views of the City and St Paul's will be compromised from Waterloo Bridge and entirely blocked along the South Bank, one of the great promenades of Europe and London's most popular walking area."

"In return for this we get a private bridge with no right of way, closed once or twice a month, and with restricted access."

The challenge is being launched in the High Court. It also raises concerns about the lack of evidence regarding the ability of the Garden Bridge Trust to fund the maintenance of the bridge over its proposed 125-year lifespan.

"The trust have produced no evidence that they can raise the £3.5 million annual running costs – four times the cost of maintaining other London bridges – and we all may end up picking up the tab," stated Ball.

"Decisions as important as this need to be taken properly and got right, for Londoners now and for coming generations," he said.

Responding to the challenge, a spokesperson for the Garden Bridge Trust commented: "This judicial review will run its course and in the meantime, the trust is proceeding with its plans on all aspects of the project, including fundraising and ongoing consultation with those affected by the development."

Initially the brainchild of British actress Joanna Lumley, the Garden Bridge is envisioned as a promenade of trees and shrubs.

Heatherwick Studio is working alongside engineers Arup and landscape designer Dan Pearson on the project. It is expected to begin construction later this year to avoiding clashing with the proposed Thames Tideway Tunnel, which is due to start in 2017.

Images courtesy of Arup.