The Union Chain Bridge over the River Tweed at Horncliffe will be closed to traffic for two weeks while site investigation work is carried out.
Funding for an £8m conservation and engineering programme for the bridge, the world’s oldest single-span suspension bridge still open to traffic, is being sought.
Scottish Borders Council and Northumberland County Council have each committed around £500,000 to the project, and site investigation work under way since Monday should yield a greater understanding of the status and condition of the bridge, on the English at-risk register since 2013, ahead of a bid for a Heritage Lottery Fund grant.
The bridge, opened in 1820, will be closed to vehicles on weekdays from Monday, July 3, for a fortnight.
Selkirkshire councillor Gordon Edgar, Scottish Borders Council’s executive member for roads and infrastructure, said: “I am pleased we are seeing progress in the bid to retain the Union Chain Bridge as the world’s oldest single-span suspension bridge still used by traffic.
“The iconic crossing has provided a vital link between Scotland and England for almost 200 years, and we want that to remain the case.
“The short-term closure of the bridge to vehicles will cause some inconvenience but will provide important information for its long-term future.”
Robbie Hunter, chairman of the Friends of the Union Chain Bridge, said: “It would be an unforgivable tragedy if we failed to save this engineering icon.”
Councillor Glen Sanderson, Northumberland County Council cabinet member for environment and local services, said: “Along with our colleagues in Scotland we are committed to safeguarding its future and status, both as a key transport link and as a contributor to the local tourism economy.
“The site investigation work is vital ahead of any major project starting and engineers will be working to keep disruption to a minimum throughout.”