Dumfriesshire, Clydesdale and Tweeddale MP David Mundell has called for urgent action after a section of the A708 collapsed into a stream near the Grey Mare's Tail.
Police have confirmed the A708 Moffat to Selkirk Road near Grey Mare's Tail is likely to be shut for some time, forcing locals into taking a significant and lengthy detour.
The MP, who has previously warned of potential problems on this busy Borders tourist route, has now written to Scottish Transport Minister Keith Brown, urging him to provide support to Dumfries and Galloway Council.
“This road must re-open as soon as possible with the inconvenience of this major detour to locals and visitors alike kept to a minimum,” said Mr Mundell.
He added: “Sadly this closure is something which has been going to happen for some time and I have repeatedly called for action to have this road upgraded.”
The MP, who assessed the situation on the ground on Monday morning, went on: “The A708 is a direct link route between Dumfries and Galloway and the Scottish Borders and I and others have expressed concerns in the past about the condition along stretches of the scenic route. Sections of the road have collapsed before with barriers put in place, effectively creating single-file traffic.
“It is now clear that repairs and upgrades are beyond the resources of the local councils and that is why I have called on the Scottish government to get involved. This is an important A class road and one essential to tourism, transport and local people, therefore it is imperative it is fixed as soon as possible.
“Fortunately no one was injured in this incident but without investment there could be other similar incidents. It is not enough just to repair this section - there must be a longer term upgrade of this route,” said Mr Mundell.
The collapse is thought to have been the result of subsidence caused by poor weather conditions.
Police have set up a diversion via Blyth Bridge, Peebles and Innerleithen and advised motorists to expect longer journey times than usual.