A huge blaze at a listed Victorian building is being treated as deliberate by the fire service.
The disused Trowse Pumping Station erupted in flames on Sunday night.
The roof was severely damaged, with aerial platforms used by the fire service after the blaze broke out at around 9pm and crews were on site until just before 5am on Monday.
After being delayed in its investigations due to the site being unsafe, the Norfolk Fire and Rescue Service now believes the fire was started intentionally.
A spokeswoman for the service said: "The evidence we have supports our finding is that the most likely cause of the fire is deliberate action, but we cannot speculate on any motive."
The blaze at the Trowse Pumping Station joins a spate of similar fires in disused buildings in the Norwich area.
The fire service has handed its findings on to the police who will now investigate the cause of the fire further.
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