Guided weapons used by UK forces are tested on the islands
Concerns are mounting about the future of 250 jobs linked to the testing of missiles on the Western Isles.
Storas Uibhist, the community-run landowner on South Uist, said there was speculation locally that jobs could go.
Sites on South Uist, Benbecula and Hirta on the St Kilda archipelago test and track guided weapons.
The Ministry of Defence (MoD) said an announcement concerning the sites would be made on Wednesday. It refused to comment on speculation about jobs.
It denied the sites would be mothballed.
The sites are run on the MoD's behalf by defence company QinetiQ.
Guided weapons are fired out to sea from South Uist and tracked by stations on Benbecula and Hirta.
Huw Francis, of Storas Uibhist, said any job losses would have an impact on the islands' economy following recent cuts in the construction industry.
He said: "There has been a lot of speculation about the facility for quite a long time.
"QinetiQ is one of the largest employers on the islands and they are skilled jobs which are in short supply here."
Storas Uibhist led the community buy-out of South Uist Estate and manages 93,000 acres of land covering almost the whole of the islands of Benbecula, Eriskay and South Uist, as well as a number of other small islands.
SNP MP Angus MacNeil said he was seeking clarification from the UK Government and QinetiQ on what was planned for the sites.
He said: "If it is confirmed this week that there are to be major job losses in the defence sector in Uist, then the UK Government are dealing a very heavy blow to the islands.
"If the UK Government has any commitment at all to supporting island jobs and maintaining island populations, they must understand that."
The site on South Uist was formerly used by the Royal Artillery to test weapons.
In 2004, the MoD instructed an environmental survey to be carried out to assess any potential risk from cobalt-60, a radioactive material used to help gauge the performance of the weapons.
Potatoes, lamb flesh and rabbit meat were tested.
The survey concluded that the radiological risk of consuming the foods to the public was negligible.
Islanders' views on the establishment of the range were also the subject of a 1961 documentary, Rocket Range Benbecula.