Leamington lift bridge in Edinburgh will be shut for months, blocking access to the Lochrin basin at the capital end of the Union Canal.
Cash-strapped Scottish Canals said damage to the 120-year-old bridge would cost some £250,000 to fix.
The Scottish Government-funded public body said: “Deterioration has reached such an extent we do not consider the structure to be safe to operate and presents a risk to both boaters and the public”.
The move follows the closure of four opening bridges that has restricted boat movements on the Forth & Clyde Canal, which connects to the Union Canal at the Falkirk Wheel.
Twechar in East Dunbartonshire and Bonnybridge, near Falkirk, shut in January, followed by others at Knightswood and outside Scottish Canals’ HQ in north Glasgow.
In a further blow, a lock gate on the Forth & Clyde in Falkirk was severely damaged by a hire boat last month, closing the east end of the canal.
Keep Canals Alive (KCA), which was launched in January to campaign for the canals, fears the worst because of Scottish Canals maintenance backlog and the fragility of the 200-year-old infrastructure.
It comes despite the two “Lowland” canals being reopened in 2001 at a cost of £78 million, with several filled-in sections being restored.
KCA chair Pat Bowie said of the Leamington bridge closure: “We are afraid this latest structural disaster could be the straw that breaks the camel’s back and a decision is taken somewhere to shut down the Lowland Canals as a whole.
“It is surprising that only now, 17 years after the re-opening of the canal, the infrastructure of the bridge looks as if it could be beyond repair and therefore saving.
“It could mean the iconic lift bridge will disappear from the landscape of Edinburgh.”
Scottish Canals director of infrastructure Richard Millar said it aimed to get all five bridges in use by next spring.
He said the Leamington bridge was regularly inspected and maintained.
“However, a recent detailed internal examination showed some elements were suffering from substantial corrosion, significantly impacting the structural integrity.”
Mr Millar said: “With a £70m backlog and a shortfall each year of £6-9m, we have to direct our spend according to safety priorities.”
A spokesman for Transport Scotland, which provided an extra £1.6m in June to repair Twechar and Bonnybridge bridges, said: “There have ongoing discussions with Scottish Canals about their asset management plan to understand the scale of the financial challenges ahead.”