"Disgraceful" thieves tore up paving stones from the grounds of an abandoned museum.
Burglars broke into Church Farm House Museum, in Greyhound Hill, Hendon, overnight and stole ten York stone paving flags.
The slabs are worth £70 each and police are currently awaiting further details about the incident.
Barnet Borough Council has been unable to secure a buyer for the Grade II* listed building since it closed in March 2011, much to the dismay of campaigners.
Former curator Gerard Roots said: “I am upset to hear this – the people who did this are crooks, they are disgraceful.
“The building is starting to look dilapidated and neglected and that means problems are more likely to occur. It’s an easy target for thieves. It’s awful, but it’s true.
“Even though the museum is no longer my responsibility, I still care about it. It shows how vulnerable the building is now it’s closed.”
Mr Roots is standing as Labour candidate for Hendon ward and if elected, says he will ensure the council maintain the building.
He added: “It’s a great shame – it’s an important part of Hendon life. It’s one of the most important buildings the council owns.”
Negotiations to sell the site to Middlesex University are ongoing, although a formal lease is yet to be signed.
Until then, the council spends £85 a day on employing security guards for the site.
The old house was once home to the Dunlop family, who lived there from 1869 to 1943, and was opened as a museum in 1944. When it closed, the authority auctioned off its artefacts making a profit of just £17,380.
English Heritage has warned it could be placed on the ‘at risk’ register if it is still empty by 2015.
GLA member for Barnet Andrew Dismore said: “The council should be ashamed of themselves.
“They will have to replace the missing flags at no doubt considerable expense to the council tax payers of Barnet, but it is difficult to see how new stones will be able to replicate the appearance of the stolen old ones.”
Councillor Daniel Thomas, deputy leader of Barnet Council, said: “It is important that a long-term and sustainable outcome for this building is reached and lease negotiations have been on-going.
“Proposals will come before the regeneration, assets and growth committee under the new council in due course.
“In the meantime, security will continue to be provided for the site. Of course, one does suspect that the thieves would have struck regardless of who uses the building in the daytime.”