The current phase of engineering works to strengthen the 1.9km viaduct that supports the railway track began in April 2021, with the aim of returning the funicular to service early next year.
However, despite strong progress made in tough conditions, HIE has concluded that the planned timescale now needs to extend into the second half of 2022.
The mountain resort will stay open throughout winter, with skiers and snowboarders able to access the slopes using nine ski tows that will run every day when weather conditions are suitable.
According to the development agency, several factors have combined to lengthen the funicular reinstatement programme.
These include technical challenges associated with designing and implementing effective solutions to an existing structure, as opposed to a new build.
In addition, progress has been hampered by COVID impacts and by extreme weather, particularly blizzard conditions in the spring.
Extra effort has also been required to source vital construction materials that are currently in short supply across the UK.
HIE interim chief executive Carroll Buxton said this combination of challenges had made rescheduling the works inevitable.
"Cairngorm Mountain Estate is a great visitor attraction that plays an important role in the local economy.," she said.
"The funicular has been a key element of what draws people to Cairngorm and it will be again, once these works are completed.
“We've always been upfront in describing the reinstatement programme as one of the most complex and difficult civil engineering projects in Scotland.
“As well as managing technical challenges, the team has to take great care to protect the mountain environment, and deal with difficult terrain and extreme weather conditions that can quickly become hazardous.
“The original schedule was very ambitious, even without the impacts of the pandemic lasting as long as they have and before the more recent problems in sourcing construction materials of suitable quality.
“With limited time available to complete these works before winter, postponement has become inevitable."
The funicular opened in 2001 but was taken out of service in 2018 after an engineers' inspection identified structural defects that raised safety concerns.
More detailed inspection and design work followed before the decision to reinstate the funicular was announced last October by the Scottish Government, as part of a £20.5m business case that includes other priority investments to strengthen Cairngorm Mountain’s year-round appeal.
The business case assessed alternatives to reinstatement before reaching its conclusions, including removing the structure, and replacing it with alternative uplift.
“We’re now working with our designers and contractors on a revised schedule for the remainder of the project,” Ms Buxton added.
“Harsh mountain conditions mean that all work will need to halt over winter and resume next spring. To ensure safety, there also needs to be a period of testing and certification at the end of the programme, so our current expectation is that the funicular can be relaunched in autumn 2022.
“Clearly, a longer timescale will also have a financial impact. We’ll be reviewing costs very carefully and will publicly confirm a revised budget as soon as it’s possible to do so.”
Confirming its commitment to the mountain resort, HIE has also approved up to £780,000 for an extensive refurbishment of the Ptarmigan building that sits on the plateau and includes the UK’s highest restaurant, a shop, exhibition space and viewing platforms.
Following competitive tender, resort operator Cairngorm Mountain (Scotland) Ltd (CMSL) has awarded a contract for the first phase of the Ptarmigan works to Simpson Builders Ltd, based in Beauly. The refurbishment is due to start at the end of August and be completed by the end of 2021.
