The Menai Suspension Bridge, one of the links connecting Anglesey to mainland Wales, is to be closed immediately because of a “structural issue”.
The Grade 1 listed landmark first opened in 1826.
It was built by Thomas Telford and spans across the Menai Strait.
The Welsh Government said an issue was found during an inspection and the bridge is likely to remain closed until the New Year while repairs are made.
Drivers will be able to still use the A55 Britannia Bridge to make crossings. The Government added it will "increase resilience" on that bridge as it often closes to high-sided vehicles during bad weather.
The Menai Bridge was the world's first suspension bridge. It is 1,265 feet long with its roadway set 30m above the water to allow tall ships to sail beneath.
The bridge will be closed to vehicles from 2pm on 21 October but will be the footpath will remain open to pedestrians and dismounted cyclists.
A short while after the announcement, traffic was reported to be building already.
A spokesperson for UK Highways A55 said, “While this issue will cause disruption, we must act in the interest of public safety.
"We are currently peer reviewing the findings that led to the recommendation of closure and assessing all available options to reopen the bridge as soon as possible so people can get back to using the bridge regularly.”
Deputy Minister for Climate Change with responsibility for transport, Lee Waters added, “This urgent work is being carried out for public safety, unfortunately it is unavoidable, but we are fully aware of the implication this will have for people in the local area.
“We are working closely with UK Highways to ensure this work is carried out safely and as quickly as possible with minimal disruption to the local community.”