Conservationists have asked one of the country’s leading lawyers to review the legal case for the controversial dams project on Hampstead Heath.

Heath bosses are drawing up plans for a £15million upgrade of the dams to safeguard against flooding from freak tropical storms. The City of London Corporation, which manages the Heath, claims it is bound by law to carry out the work. But the Heath and Hampstead Society is questioning the scheme’s legal premise.

It has employed a top QC, specialising in environmental law, to produce an independent view of the “complex legal issues” involved in the dams project.

Society chairman Tony Hillier said: “We’ve always been in debate about the legal advice given to the City. If it turns out that we are correct in believing that the City has not been well advised, then obviously we will tell them where they need to think again. Then it’s up to them.” He added: “Our hoped- for outcome is that there will be no visible changes to the look and environment of the Heath. That’s what we’re here to do, preserve its wild and natural status.”

In 1975 water cascaded to parts of Gospel Oak and Dartmouth Park, causing more than £1million worth of damage.

But the society claims this was due to a faulty sewage system rather than a failure of the dams system. The ponds have only overflowed once in recent years.

The society is questioning whether an act to preserve the appearance of the open space, known as the Hampstead Heath Act 1871, should trump the Reservoirs Act 1975, which requires the City to safeguard against flooding.

If this proves to be the case the society will argue that this should be taken into account when chief architect Peter Wilder draws up the final plans for the scheme. The first sketches are expected to be unveiled in May or June.

Mr Hillier said: “The difficulty is that the Reservoirs Act states that it’s really up to the engineers to decide what, if anything, needs to be done to strengthen reservoirs in, and this is the key phrase, ‘the interests of safety’.

“There are three issues. Firstly what does ‘in the interests of safety’ mean? Secondly, who decides – should it be an engineer unguided by wider statutory and legal consultation? And thirdly, what are the approved methods and methodologies that should guide the research and analysis for whoever makes the final decision on what ‘in the interests of safety’ means. What is the approved methodology, modelling and measuring technique in order to assess the risk?”

The City of London claims that under new laws proposed in the Flood and Water Management Act 2010 it will be legally bound to undertake the works.

Jeremy Simons, chairman of the City’s Hampstead Heath Management Committee, said: “The City of London has a statutory obligation to carry out this work, and is following the current guidance set by the Institution of Civil Engineers in relation to reservoir legislation and dam safety.

“Our challenge is now to carry out the necessary works to the dams and ponds while still preserving the unique natural environment of Hampstead Heath.”

The project stalled briefly last week as the City struggled to find a national contractor to take on the project. But Heath bosses have resolved to approach smaller firms to carry out the work.