Rail ticket prices in Brighton could rise because the cost of cleaning off graffiti has escalated.
The graffiti often put trains out of service
South Central Trains says damage done to trains and stations by two graffiti gangs in Brighton is costing almost £3m a year to clean up.
Now the company has warned that the cost of the damage may have to be passed on to passengers.
In the past few months, trains have been targeted at every railway siding between Brighton and Gatwick.
David Haynes, head of security at South Central Trains, said: "It is annoying and obviously these sort of costs get passed on to the customer.
"The real thing that annoys us is when we are not able to deliver a train the following day to the passengers that are expecting it."
He said sometimes it would be dangerous for the trains to be used because graffiti had been sprayed on the driver's window.
Overall the company said it costs £1.7m a year to clean trains with £500,000 spent on cleaning stations.
Every time a train is put out of service it costs the company £20,000.
British Transport Police said attempts were being made to try to stop the vandals by using CCTV and bringing in handwriting experts to try to identify tags sprayed on trains.