Plans to launch satellites into space from a site in North Uist threaten a "pristine section of wild Atlantic coastline", say conservationists.
Local authority Comhairle nan Eilean Siar is working with Ministry of Defence contractor QinetiQ and others on the project planned for Scolpaig.
The comhairle says the site would help boost the isles' economy and could create up to 70 jobs.
But North Uist Conservation Group has raised objections.
The campaigners said the land would be lost to concrete structures and have also raised concerns about the potential impact on St Kilda.
St Kilda lies about 40 miles (64km) west of North Uist, the nearest inhabited place to the archipelago.
The conservation group is calling on islanders to send written objections to the local authority.
'Usual public way'
The comhairle said it was aware of the concerns, but also of support for the project within the local community.
Public information meetings on the plans are due to be held in mid-August.
A council spokesman said: "The planning application has been lodged with the comhairle and will be determined in the usual public way, with the opportunity being given for all interested parties to make representations.
"The planning process itself is inherently consultative, as it gives the requisite levels of detail of the proposed development, and invites comments on them.
"The application will be determined in the same way, and using the same professional standards, as any other planning application."
Most northerly island
The comhairle has agreed to invest about £1m to purchase the land needed and test launches could be carried out later this year.
QinetiQ, which operates the nearby Ministry of Defence Hebrides Rocket Range, is a partner in the project.
Highlands and Islands Enterprise (HIE) and consultancy Commercial Space Technologies are also involved.
In March, three companies were appointed to work on the design of the spaceport proposed for land on the Melness Crofters Estate in Sutherland.
Like the North Uist project, the site would be used for vertical launches of small satellites.
HIE is heading up the £17,5m Space Hub Sutherland project.
Unst, Scotland's most northerly island, has been proposed as the location for the Shetland spaceport.
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