The Forth & Clyde Canal is to close indefinitely in two places because of "unsafe" bridges, The Scotsman has learned.
The move by Scottish Canals will halt through traffic on the waterway between Bowling on the Clyde and Grangemouth on the Forth.
The Inland Waterways Association condemned the move as "unacceptable".
Boaters fear it will prevent dozens of yachts from transiting between the east and west coasts if the closure continues through the summer.
No vessels will be able to pass lift bridges at Twechar and Bonnybridge, which cannot reopen until repair funding is found.
The Lowland Canals Association said the bridges were "very unsafe".
Scottish Canals has told it: "With the technical problems that we have identified, we have no option but to temporarily close these bridges until funding can be identified to facilitate either comprehensive repairs or, preferably, a long-term upgrade to their operating mechanism.
"This unfortunately means we will need to temporarily close the Forth & Clyde Canal to through traffic.
"We see this as a temporary closure and we will work hard with the Scottish Government and partners to identify the necessary investment to get these bridges operation as soon as possible."
The Inland Waterways Association (IWA), which campaigns on navigable waterways, called for the repairs to be made "as a matter of urgency".
It said Scottish Canals had a statutory requirement as navigation authority to keep the canal - restored in 2001 after 40 years' dereliction - in full working order.
Chairman Ivor Caplan, said “IWA wishes to see through navigation on the Forth & Clyde Canal reinstated as soon as possible.
"We are concerned about the viability of the hire boat trade with only half the canal available, the impact on those boaters based at Auchinstarry Basin, and increased future maintenance costs as nearby locks and other structures will suffer through lack of use if these bridges are closed for any significant length of time.”
Scottish Canals said Twechar bridge would be the priority for reopening following consultation with canal users.
It has offered to cut craft licences for vessels between the bridges to £1 a month from today.
The body is also considering opening the bridges for a day to enable vessels moored between them to be moved.
Infrastructure director Richard Millar said: “Boat transits through Twechar and Bonnybridge lift bridges on the Forth & Clyde Canal have been temporarily suspended until we can work up the technical solutions and source the funding required to implement the necessary repairs to the structures.
"We will always err on the side of caution in matters of public safety.
“The restrictions will only temporarily impact our transiting customers, of which there are around 50 per year.
"The vast majority of canal users, from runners and walkers to cyclists and kayakers, will be completely unaffected.
“We will keep our customers fully informed of this evolving situation and will endeavour to take any reasonable steps to ensure each of our boaters is able to get to where they need to be and enjoy the season on the Forth & Clyde Canal.”
Scottish Canals said at Bonnybridge the vertical lift bridge's hydraulic system was not keeping the raised deck level.
It said: "This means when lifting and lowering the deck, there is a possibility of damage to the bridge which is likely to result in extended opening time of the bridge, to re-level and lower the bridge, which would significantly delay the re-opening of the road to traffic.
"This would mean an eight to ten mile diversion for emergency vehicles, cars and buses."
It follows the bridge being repaired in 2016 after a steel plate protecting the reinforced concrete bridge deck was damaged because of the bridge lowering out of balance.
The body said at Twechar hydraulic bascule bridge, a bearing on one of the lifting cylinders has shown "concerning signs of wear and movement".
It said: "The extent of the wear has continued to a point where Scottish Canals has been forced to take the bridge out of service.
"Scottish Canals is now working with specialist contractors to investigate this connection, assess and have critical parts available to return the bridge to service based on satisfactory findings of the inspection.