The 50-metre bridge across the River Tawe is being refurbished off-site by Afon Engineering in Swansea as part of a council regeneration of the Hafod-Morfa Copperworks area. The aim is to re-install the restored structure next year.
Robert Francis-Davies, the council's cabinet member for investment, regeneration and tourism, said: "Work is essential at this time to prevent further decay and risk of loss of this Grade Two listed bridge which is also an officially scheduled historic monument. Any further delay would result in the loss of this valuable heritage that forms a critical part of Swansea's story.
"Our work on the bascule bridge will complement the work we are doing to develop a world class tourist destination at the copperworks; this has attracted National Lottery Heritage Funding of £3.75m to restore the Powerhouse for future use as a Penderyn Distillery visitor attraction."
The steel and timber structure, also known as the Morfa Bridge, was built in 1909 to strengthen Swansea's world-famous copper industry. It provided a rail link between the Morfa and Upper Bank works.
A large part of the deck could originally be lifted using a hand-cranked mechanism to allow boats up and down the river. It was a key river crossing for the copperworks and the new work will complement other local transformation such as that with Penderyn Whisky.