The gateway is the restored inner gatehouse of the ruined Reading Abbey
Reading's historic Abbey Gateway has been shut after stones fell from the building and it was deemed unsafe.
The council said it was following advice from a structural engineer who inspected the gateway after decorative stone and sandstone fell from it.
The access route under it has now been closed while more detailed investigations take place.
Temporary fencing has been put up around it, directing pedestrians to an alternative route around the building.
Tony Page, deputy leader at Reading Borough Council, said: "We cannot take any risks with public safety either within or in the vicinity of the gateway, and have therefore moved swiftly to erect the necessary scaffolding and security fencing.
"We will be working closely with neighbours and the current occupants to ensure as little disruption as possible."
The gateway is the restored former inner gatehouse of Reading Abbey, of which only ruins remain in the centre of the town.
The abbey was founded by Henry I in 1121 and the gateway is one of only two of its non-ruined buildings remaining.