More than 300 people have signed a petition to fight a “monumental” development in the heart of Hampstead.

Parents from two primary schools and residents around New End are rallying to oppose plans for 15 homes, in what campaigners have branded a “mass overdevelopment” in a fragile conservation area.

Residents claim the excavations for a three-storey basement and building work on the New End Nurses Home would see a fleet of 36-tonne lorries rumbling through narrow Hampstead streets, with roads already weakened by tunnels and underground streams.

Hampstead Town Cllr Chris Knight said: “These trucks will just be massive for Hampstead’s fragile roads.

“They’re going to shake these buildings to bits and many of them are 200-years-old and made from clay from the flats on the Heath – they’re very biscuity vulnerable buildings.”

Magdeleine Kendall, who is spearheading the campaign, claims 1,200 lorry trips will be necessary to clear 43,000 tonnes of rubble from the site, if planning officials give the development the go-ahead next month.

Campaigners are raising a fighting fund to pay for an independent engineer’s report to challenge the developer’s survey of the area.

Mrs Kendall, of Hampstead Square, said: “It’s not just a small basement, this is massive.

“Our argument is that we are not totally against development, but we would like it to be restricted to the original footprint of the building.”

Two months ago Camden Council made a landmark ruling giving the green light for a vast basement to be built in Templewood Avenue, which ballooned beyond the original footprint of the multi-million pound mansion.

Planning committee member Cllr Matthew Sanders warned it would set “a dangerous precedent” for future basement developments.

Camden’s planning guidelines say larger schemes must show evidence that they “do not harm the built and natural environment or local amenity”.

Jane Henderson, of Hampstead Square, has been offered �50,000 compensation should developers damage her home, which she described as a “pathetic sum”.

She said: “That �50,000 is hardly going to cover the cost of the paintwork, let alone the underpinning of my home.”

Conservation watchdog the Heath and Hampstead Society has also warned of the dangers of disturbing the fragile surface soils which could lead to flooding.