A developer has been threatened with legal action after builders set to work on a historic Hampstead home without permission.

The town hall has issued a warning to Karawana Limited which has plans to demolish an old nurses home in New End after two builders – armed with shovels and a pickaxe – started to illegally dig a trench under a grade II listed property which backs onto the site.

Camden Council told the developer last Friday that it could face being prosecuted under the Police and Criminal Act should it try to carry out illegal works again.

Homeowners, who are already concerned about proposals for a three-storey basement under the nurses home, are now fearful the developer will “ride roughshod” over planning procedures and promises given to the community, should Karawana Limited be given the green light to work on the site.

Jane Henderson, owner of the listed property in Hampstead Square, said: “This is just indicative of the way the developer behaves and it is very worrying. Once they have planning permission it will be impossible to control them.”

The builders dug a trial hole next to the listed party wall to test the foundations.

Martin Hicks, who is leading a community campaign against the development, said: “By sending in builders to illegally dig a trial pit alongside the buttresses, the developers rode roughshod over assurances they gave to Camden and to us, sidestepped planning procedures and put at risk an 18th Century wall and listed building.”

Hampstead Town councillor Chris Knight said: “These developers are just seriously taking the mickey. It has become a war of attrition, pushing that envelope as fast and hard as they can.”

In a long-running campaign residents have gathered hundreds of signatures for a petition against previous schemes and now planning officials are considering fresh plans to demolish the 74-bed hostel. Proposals have been submitted to build 17 new homes on the sight – including a basement car park – which used to house staff working at New End Hospital which closed in 1986.

No date has yet been set for a planning hearing.

No-one from Karawana Limited was available for comment.