A government report brought to light this week has shown there is no justification for the controversial ponds project, conservationists have claimed.

Campaigners are taking a desperate last stand against the £15million dams engineering scheme, which would drastically alter the 300-year-old ponds.

New government guidance on small reservoirs – which applies to all but three of the Heath’s ponds – states that they will not now be subject to strict safety regulations.

Opponents of the project say that there is therefore no reason why the ponds project should still go ahead.

Heath and Hampstead Society vice-president Helen Marcus said: “To me, it takes away the whole reason the City of London claims they had to do this.”

The Dam Nonsense campaigners made the claims as construction works began on Monday on the scheme to raise the dams by as much as 2.5m.

The City of London Corporation, which runs the Heath, argued that the new government guidance does not affect the ponds project in any way.

The report states that safety measures, such as strengthening dams, are only required for large reservoirs under the Reservoirs Act 1975. It goes on to add that plans to introduce new legislation to cover smaller reservoirs have been put on hold over concerns about unjustified cost until the next Parliament.

There is currently “no evidence base” for the changes to the law, the report states.

Only three of the Heath’s ponds are classed as large reservoirs – the Men’s, Model Boating Pond and Hampstead No 1. However, the dams project will involve altering all of the Heath’s ponds. An unsuccessful judicial review to halt the project last year hinged on the reservoir safety legislation.

Mrs Marcus said: “Most people feel that the risk to the three ponds has been grossly exaggerated anyway.”

A spokesman for the City of London said: “The City has chosen to spread the required safety works across the ponds in the chains rather than focusing heavy engineering on just the three large raised reservoirs.

“This allows us to use a ‘soft engineering’ approach, safeguarding the Heath’s landscape and avoiding any big concrete dams in the project. The work taking place is vitally important to safeguard the downstream community.

“The scheme also future proofs the Heath from engineering works required when the 2010 Act is fully introduced.”