ECO-warriors are threatening to wage a sit-in protest – in the treetops – in a last-ditch attempt to stop a secret garden being built over.

The campaigners have threatened to camp out in makeshift treehouses after Islington Council granted permission for 143 flats to be built on the former UK headquarters of the charity Action for Children.

The HQ in Highbury Park, Highbury, is home to an area of historic urban grassland that campaigners say is “teeming with wildlife”.

Islington Green Party spokeswoman Emma Dixon, who is spearheading the protest, said: “Planning permission has been granted but nobody has started digging up the garden yet so until they start digging, there is still hope.

“Residents have been coming up to me in the street saying they want to do something more direct – maybe picket Action for Children’s offices or maybe build treehouses in the garden.”

Action for Children, formerly NCH, has moved to bigger and more modern premises outside Islington after 80 years in the borough – and is selling the land to developers to help finance the move.

On October 11, after months of wrangling, Islington Council granted permission for the “secret garden” and all the buildings on the site – except for the 1850s–built Loxford House – to be bulldozed to make way for 143 flats, a children’s nursery, and community space.

The go-ahead was given after Action for Children agreed to up the proportion of affordable homes from 40 to 45 per cent.

Councillors have also insisted than an 80 square metre biodiversity area is created to replace the historic grassland. There will also be a new landscaped open space in the middle of the development.

Councillor Robert Khan, chairman of Islington Council’s planning committee, said: “The committee listened carefully to all evidence and decided that there was significant public benefit in the creation of much needed new affordable family homes in Islington.”

Campaigners are hoping that Action for Children can be persuaded to rework the scheme to salvage the secret garden.

But the charity insists that if the scheme were redesigned, it would significantly reduce the number of homes that could be built, would cause a six to nine month delay, and would jeopardise funding for the project.