A landmark battle over the future of Hampstead Heath’s 300-year-old ponds is due to begin at the High Court today.

Conservationists have taken controversial plans to drastically alter the much-loved ponds to judicial review in the hope of halting the £15million project.

They are battling to save the Heath from being “permanently disfigured” by two years of construction work to build a huge new dam in the Heath’s Catchpit area, raise existing dams by up to 18ft and transform the landscape around some of the ponds.

They claim the two-day hearing will be of national importance as it puts reservoir safety legislation to the test.

Bosses at the City of London Corporation, which manages the Heath, say the work is required by reservoir laws – to prevent the dams collapsing and triggering a fatal flood if a “catastrophic” storm hits.

But the Heath and Hampstead Society, which is mounting the challenge, insists the City has gone far beyond the requirements of reservoir rules and failed to take proper account of the Hampstead Heath Act, which gives statutory protection to the wild landscape.

David Lewis, of Protect Our Ponds, one of the groups supporting the society, said campaigners are “cautiously optimistic”.

“We’re hoping it will show up the inadequacies of the reservoir safety legislation,” he said.

“I think there’s a good chance of success, though you can’t tell how these things are going to turn out.”

In papers lodged with the court, the society gives two main legal grounds for challenging the project:

n The decision to proceed is based on an “erroneous interpretation” of what measures are required in the interests of safety.

n It is based on an irrational and/or unlawful approach to risk.

Thousands of people have objected including the vast majority who responded to a public consultation carried out by the City.

The work is currently due to begin in January and is estimated to last nearly two years, until October 2016.

If it goes ahead, heavy duty vehicles including five-tonne tankers, nine-tonne dumpers and a 90-tonne crane will make thousands of trips in and out of four entrances to the Heath.

Twelve ponds across the Heath will undergo works for between four and 33 weeks each, including the three bathing ponds. The ladies’ pond will be shut from mid-February 2016 until mid-May 2016.

The aim is to “virtually eliminate” all risk of dam collapse by ensuring they would withstand the “probable maximum flood”, which has a 1-in-400,000 probability of occurring each year.

A judge’s ruling is expected by the end of the year.

Chairman Marc Hutchinson has previously indicated the Heath and Hampstead Society will lobby Parliament if it loses the case.