Several high-profile construction sites, including tennis courts at Wimbledon, have been targeted by building-site intruders.
The group, which calls itself UK Urban Exploration, breaks into the sites, takes pictures and posts them on the internet.
Wimbledon football stadium, in south-west London, and the Newton building in Nottingham have also been flagged up as targets.
Part of this group, known as “craniacs”, climb tower cranes and take photographs of themselves in dangerous poses.
Several sites, including the former Millennium Dome and the Old Stock Exchange development in the City of London, have been broken into and pictures have been posted on the website 28dayslater.co.uk.
One section of this internet site suggests targets and how best to break into them.
One person advised intruders that it was easy to break into the £70m Newton building site. The person said: “Went into the site last night and it was very easy … the cameras that used to be in the yard are gone and no signs of any overnighters.”