Hampstead conservationists fear a half-acre portion of the Heath could get built on if it falls into the wrong hands at a controversial auction on Monday.

Ham & High: Harry Hallowes lived in a shack on the Heath for 20 years before gaining the land under squatter's rightsHarry Hallowes lived in a shack on the Heath for 20 years before gaining the land under squatter's rights (Image: Archant)

Harry Hallowes, who was awarded squatter’s rights in 2007 after living on the plot for 20 years, inspiring a 2017 film, left the land in his will to two charities for the homeless, Shelter and Centrepoint.

The City of London Corporation (CoL) and the 121-year-old Heath and Hampstead Society made an offer for the land, which is below the private gardens for Athlone House – but was rejected. The groups say they would have incorporated it back into the Heath had the offer been taken up.

The society’s chairman Marc Hutchinson said: “The society is disappointed that the vendors of the plot have rejected a generous offer by the City of London Corporation and have decided to auction it.

“There is always a risk that, if the land remains in private hands, it will become subjected to attempted development, contrary to Harry’s known wishes and to the detriment of the surrounding Heath.”

Planning laws prevent any development on the plot, meaning it could only legally be used as a garden if purchased by a private buyer.

CoL’s Hampstead Heath management committee chairman Karina Dostalova said: “Harry had fought developers himself so we thought he would have been happy for it to become part of the heath.

“We still fully intend to bid but, as we are a charity, we only have a certain point we can go to and we just do not know who the other bidders might be.

“Hampstead Heath has a unique character with lots of wildlife species that flourish there and you just do not know what plans a private buyer could have for that land. They could chop it all down.”

The charity, which spends more than £5m a year to maintain the Heath, said it made its original offer based on independent evaluations.

In a joint statement, the charities said they would put funds raised by the sale directly into the vital work that Harry wanted to be his legacy.