Health bosses have been forced to step up security after an outbreak of vandalism.
Yobs are believed to have caused about £3,000 damage at the headquarters of Worthing Priority Care NHS Trust in Arundel Road over the last six months.
Acts of vandalism have included smashed windows, graffiti and damage to cars around the building site of the new Meadowfield mental health unit.
The trust has now set up extra security patrols around the area and more security fencing is expected to be installed at a cost of about £20,000.
A trust spokesman said: "It is an annoying situation that we, as a care industry, are having to take money that is directed at improving and caring for people and use it for security like this instead.
"We have also had to take people away from other areas of the site where they are working to help improve facilities for patients and get them to work on broken windows instead."
The trust believes it is a group of young people that have been regularly hanging around the building site area over the past few months and staff are worried the situation could get worse during the summer holidays.
It is now asking parents to be more aware of what their children are up to and to warn them to stay away from the site.
The trust spokesman added: "Apart from the damage, the building site is also a dangerous area and there is the potential for a serious accident."
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