Protesters against the controversial Heath ponds dams project are still refusing to give up the fight despite having their hopes for halting the scheme dashed this week by environment secretary Liz Truss.

As she paid a visit to the Hampstead Heath Men’s Bathing Pond on Tuesday to “see for herself and understand the background” to the situation, the MP’s department had already ruled out intervening to halt the work.

Work started on the £22 million scheme in the last month as contractors started clearing undergrowth and cutting down trees and is expected to last until October 2016.

It comes after Camden Council approved the project in January despite a 12,465-strong petition against it and over 1,000 letters of objection. Protesters claim it will now cause irreparable damage to the historic landscape and “vandalise” the Heath.

But councillors accepted the argument from Heath managers City of London Corporation that the work was required under the 1975 Reservoirs Act to prevent the dams collapsing and triggering a fatal flood if a “catastrophic” storm hits.

Marc Hutchinson, chairman of The Heath and Hampstead Society, which has been leading the protest for the last three years, had written to Miss Truss asking her to use her exempting power under the act to exclude the ponds by reclassifying them as “ornamental ponds”, which would mean the work was not necessary.

He received a reply last week from Dan Rogerson MP, under secretary of state with responsibility for ponds, which said that the secretary of state did not have the power under the act to exempt specific reservoirs.

He wrote: “I do not consider it appropriate to exempt the Hampstead Ponds from being treated as large raised reservoirs, for the purposes of reservoir safety regulations.”

Despite this, Miss Truss still arrived to be shown around the ponds by Hampstead and Kilburn’s prospective Tory MP Simon Marcus and Mr Hutchinson and to listen as they tried to persuade her to intervene.

She said: “I wanted to come and see it for myself. I wanted to see the situation and understand the background.

“It is clearly a very complex issue. But we are at a very late stage in the process. I can completely understand the concerns of local residents and this is a beautiful landscape.

“But it has been to court and a decision has been made by the local council. I think there is a limited amount that can be done at this stage.

‘‘But I have come here today to listen to local people and see what they have to say. But I cannot make promises”

Mr Hutchinson said after Miss Truss’s visit that he disagreed with the legal assessment in Mr Roger’s letter and that the Secretary of State did have powers to intervene.

He said: “I will be challenging her in writing on that.”