The Cairngorm funicular railway is to remain out of action through the summer and possibly longer, Highlands and Islands Enterprise (HIE) has said.
Engineers have raised safety concerns about the piers, beams and foundations on the railway, which has not operated since September last year.
HIE owns the funicular and the wider Cairngorm Mountain ski area near Aviemore.
The snowsports centre remains open for business.
Concerns about the railway structure were raised last year and investigated between September and December.
HIE said engineers had concluded that the structure did not present an immediate danger to the public.
But it added that the safety margin was "lower than desirable" and a series of measures should be put in place to address weaknesses before resumption of the service.
Work is required to strengthen the piers, beams and foundations, and install new bearings with higher load and movement capacity.
HIE said it was exploring the scope and cost of this work.
What is the Cairngorm funicular?
It is the UK's highest railway.
Built at a cost of £26m and opened in 2001, the railway connects a base station with a restaurant 1,097m (3,599ft) up Cairn Gorm mountain.
Thousands of people use it, including skiers and snowboarders during the ski season to access slopes.
Many of these visitors also spend time in and around Aviemore.
At the same time, HIE has commissioned an independent peer review of the engineers' report, in line with industry best practice for complex investigations.
Work on the structure of the funicular can only be carried out during summer months when the mountain is more accessible to contractors.
HIE said this would mean the funicular remaining out of service for the remainder of the current winter season and throughout the summer months and possibly beyond.
To help boost the winter snowsports season, snow-making equipment has been in use since December to provide snow for the centre's lower slopes.
Ski tows provide uplift to higher parts of the mountain when there is enough natural snowfall and conditions are favourable.
HIE's subsidiary operating firm Cairngorm Mountain (Scotland) Limited (CMSL) has also started to develop a range of alternative options to offer summer visitors.
Susan Smith, HIE's head of business development, said: "We have considered carefully the implications of the investigation and concluded it will take considerable time to design, procure and complete the necessary works.
"Our goal is to have the funicular up and running as soon as is safe to do so, but in the meantime, we anticipate it will remain out of service throughout the summer months and possibly beyond. We will provide further updates as things progress."
Ross Harris, interim chief executive of CMSL, added: "Cairngorm Mountain is an outstanding all year-round tourist attraction.
"The funicular is of course a unique feature of Cairngorm, but there is so much more to experience.
"We are working hard to ensure that we continue to provide the best experience possible for our snowsports customers who can access the mountain using our network of tows and we are confident we can build on an already attractive offering for our summer visitors."
The Save the Ciste Campaign, which has called for the reinstatement of ski tows in Coire na Ciste in the Cairngorms, said it would expect the cost of repairing the funicular to be "substantial".
Alan Brattey, the campaign's chairman, said the long suspension of operations was not a surprise.
He said: "It was fairly obvious it was going to be a lengthy closure when it could not be repaired before this winter.
"It's clearly going to be expensive to repair since it is a long project and given what they are saying is wrong with it."
"Going forward, the maintenance costs in the future are very likely to be significant, more than anyone could have originally envisaged."
Mr Brattey said the expense of replacing and maintaining the funicular could lead to replacing it with a suitable, alternative form of uplift being an option.