Defiant campaigners have vowed to continue fighting the Hampstead Heath ponds project by any means possible after their long-awaited legal challenge was rejected by the High Court.

The Heath and Hampstead Society and its supporters insisted the battle to protect the Heath was far from over after their judicial review claim was thrown out on Friday.

They are now considering an appeal and have pledged to lobby Camden Council and call on environment secretary Liz Truss to halt the controversial dam-building scheme.

Representatives of the Heath and Hampstead Society, the Highgate Society and the three pond swimmers’ groups mounted a united front as they staged a mini-protest on Tuesday.

They gathered in the rain by the Model Boating Pond, set to see some of the most drastic changes.

Mark Barclay, a committee member of the Highgate Men’s Pond Association, said: “We are resolute and determined and will use any legitimate means to prevent this dam nonsense.”

David Lewis, of the Protect Our Ponds campaign, part of the Dam Nonsense campaign, said: “Despite this unfortunate verdict, we must continue to fight these pernicious proposals.”

Rachel Douglas, chairman of the Mixed Pond Association, said the judicial review would have been won in a “court of common sense” and added: “We’re not just going to lie down or go away.”

Mrs Justice Beverly Lang dealt a major blow to their hopes when she ruled in favour of the City of London Corporation’s proposals.

Her decision removed one of the major hurdles that has been holding up the scheme, though the City still has to win planning permission from Camden Council.

The case hinged on the standard of safety used in the City’s designs. Expert engineer Dr Andy Hughes has ruled that three of the ponds are reservoirs of the highest risk category – due to the deadly threat their collapse would pose to downstream communities. This means the dams must be able to withstand the most extreme storm imaginable.

Mrs Justice Lang found that the City was right to follow Mr Hughes’s advice and attempt to “virtually eliminate” the risk of failure even in a catastrophic storm with an astronomical 1-in-400,000 chance of happening each year.

She said: “The escape of thousands of gallons of water would be likely to have a catastrophic effect on people and property situated below.

‘‘In my judgement, the [City’s] decision to approve the proposals and seek planning permission for them was lawful... [The City] was entitled to conclude that preventative action should be taken now.”

On Friday, Heath and Hampstead Society chairman Marc Hutchinson remained hopeful.

He said: “The most interesting thing which came out of the trial is that the City accepted, as we believed was the case, that Camden Council could require the City to make changes to its designs.

“Camden Council is not bound by the Reservoirs Act and we will be lobbying the council further.

“We would ask all those who have not yet written objections to Camden Council to do so, as it’s clear the council has a role to play in the final outcome.”

Bob Warnock, superintendent of Hampstead Heath, said: “This decision shows that our interpretation of the law and guidance is correct – and it means that the Heath’s landscape can remain at the heart of the scheme.

“We are protecting the Heath, improving its ecology and water quality, whilst ensuring that the earth dams are strong enough to meet modern safety standards.

“We have a legal duty to make sure the dams on the Heath do not fail, an event which would have serious consequences to the residential community downstream.”