Derelict buildings known locally as Knightstone Island in Weston-super-Mare are being closed to the public following safety concerns.
North Somerset Council and the Island's leaseholders BS (Knightstone) Ltd, made the decision after several buildings were broken into recently.
Access will be closed from 26 July and security fencing installed.
A council spokesman said: "Clearly it is important that the safety of all residents and visitors is considered."
The fencing preventing the public from getting onto the island will be situated where the access road meets the island.
Vessels' moorings
Access to the lower slipway will be maintained but there will be no access to the Marine Lake causeway.
People can still access the beach area of Marine Lake and the upper slipway.
The spokesman added: "We have taken this decision to ensure that no one comes to any harm.
"We recognise that some people may be disappointed that they can't walk among the buildings but safety has to come first."
Access to vessels' moorings will be maintained during the closure of the rest of the island.