A 14-year-old boy from south London has been charged with an offence linked to “extreme rightwing terrorism” after a police search uncovered an alleged plot to target two mosques.

The boy was arrested on 9 July on suspicion of criminal damage to a vehicle, but officers dispatched to his address found “a number of documents of concern” during a search, according to the Metropolitan police.

The terrorism charge relates to an alleged plan to target two mosques in the Sutton area of south London. The force said the affected mosques had been contacted and were being supported by specialist officers, and that they were not looking for any other suspects in connection to the plot.

After further inquiries by counter-terrorism police working with the Crown Prosecution Service, the child was charged with preparation of terrorist acts on or before 9 July, as well as racially aggravated damage to property by allegedly damaging a car window on or before 20 June, also in Sutton.

Helen Flanagan, the head of counter-terrorism policing London, said: “This is a very serious terrorism charge against a young boy and likely to be highly concerning to the public and the local community.

“We know this will be particularly concerning to the Muslim community and we are working closely with the venues affected to ensure they are kept updated and to provide advice, support and reassurance, and this will continue.”

She added: “While it is deeply concerning to see someone so young charged with offences of this nature, we are sadly seeing an increasing number of children and young people in our casework.”

DCS Nick Blackburn, who is in charge of local policing for the south London area, said: “We should not underestimate the cumulative impact of incidents of this nature on the Muslim community.

“These charges come just days after 12 people were arrested for a suspected threat to an Islamic festival in Suffolk and a man was arrested for an alleged assault outside a mosque in Leyton.

“We have been working closely with our colleagues from counter-terrorism policing London throughout their investigation and we have increased patrols in the affected areas, which we will look to continue over coming days.”

Last month, new national guidance from the Muslim Council of Britain advised mosques in the UK to carry out lockdown drills, strengthen ties with police and improve CCTV coverage, amid growing concerns about anti-Muslim attacks.

The guidance warned that Muslim places of worship faced an increasing threat from vandalism, intimidation, threats and targeted hostility.

The boy’s arrest follows a series of incidents targeting Muslim communities, including a “potential serious threat” to an event in Suffolk, racist riots in Belfast last month, and attempted firebombings, vandalism and violent attacks on mosques in Glasgow, Edinburgh, Blackburn, Manchester, Liverpool, Shrewsbury and east London.