A vehicle used by St John Ambulance volunteers in Southampton may have to be replaced after being vandalised and set on fire.
The mobile treatment centre is a regular sight at major events including country shows and parades.
St John Ambulance said the vehicle and life-saving medical equipment could cost more than £150,000 to replace.
Hampshire Constabulary said it was investigating if it was linked to a series of arson attacks during August.
The treatment centre was in a car park at St John Ambulance's premises on Dean Road when it was targeted between 17 and 22 August, the charity said.
A fire was started in its treatment area after an attempt to break in.
It is currently in the hands of insurers and will no longer be able to used at events
Regional operations manager Dan Bevis described it as "utterly mindless vandalism".
"The vehicle plays a key role in the first aid provision at events, and we're now facing the possibility that the treatment centre may have to be replaced altogether, which is likely to be very costly.
"We need active, fully operational vehicles for our volunteers, to enable them to keep our communities safe."
Last week police released a CCTV image of a man they wish to speak to after 13 arson attacks, mainly targeting cars, caravans and motorhomes in Bitterne, Peartree and Woolston between 3-24 August.