Six Royal Mail vans worth more than £90,000 have been destroyed in arson attacks.
Consignia, the parent company of Royal Mail, was forced to bring in hire vans to maintain postal deliveries.
The attacks happened in Cheapside, near Brighton station, over the past two weeks.
Three vans were destroyed in one night when flames from one spread to vehicles parked nearby.
The attacks put so many of the fleet out of action there was a threat to deliveries.
Demelza Mundy, a spokeswoman for Consignia, said: "In order to minimise any disruption to customers, additional vans have been hired for use during delivery and collection of mail.
"When the incidents happened, the vans were empty and no mail was destroyed.
"We condemn the vandals who carried out these attacks and urge anyone with information about those responsible to contact the British Transport Police."
Inspector Gary Ancell, of British Transport Police at Brighton, said: "We have had scenes-of-crime officers checking for clues and we urgently need to speak to witnesses."
Anyone with information should call 0207 391 5275.
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