Highland Council could raise an objection to plans for a major hydro-electric project using a loch described as the UK's largest operational storage reservoir.
Fearna PSH Ltd proposes re-purposing a 1950s-built hydro scheme at Loch Quoich, near Invergarry, as a pumped storage hydro (PSH) facility.
In a report, council officials said there were concerns about the development's impact on the mountain landscape and its wildlife, and rural roads due to construction traffic.
The developer said its project was "nationally significant" and would be capable of storing "vast amounts" of energy until it was needed to power more than half of Scotland's homes.
Local community councils, Mountaineering Scotland and RSPB Scotland have also raised concerns.
The Scottish government has asked Highland Council for its position on the project.
In their report to next week's south planning applications committee, officers said there were not sufficient plans to "reasonably mitigate" the scheme's effects on wildlife and geological features of special interest.
They also said there would be "significant adverse effects" on protected areas, including the Moidart, Morar and Glen Shiel Special Landscape Area.
The officials added: "The proposed development would have significant adverse impacts on the local road network, including the C1144, and the mitigation proposed does not adequately address these impacts."
They have recommended that councillors raise an objection.
The report noted Fearna PSH Ltd had held public exhibitions on its plans.
Pumped storage hydro involves two bodies of water at different heights.
The water flows from one to the other through tunnels, passing through a power station to generate electricity.
When there is low demand for electricity from consumers and/or when surplus power is available from wind farms, electricity is used to pump water from the lower level to fill a reservoir further up the hill.
The water can then be released from the upper reservoir, flowing down the tunnels to drive turbines which generate hydro-electricity. This happens at times of high demand, or when there is not enough wind to power wind farms.
Fearna PSH Ltd proposes using Loch Quoich, which holds millions of cubic metres of water and where a hydro-electric power scheme has been operational since 1955.
The project, which could be operated for 100 years if approved, would use an upper reservoir called Loch Fearna.
The developer said the scheme had been "carefully and sensitively" designed and included an "ambitious" biodiversity enhancement programme.
A spokesperson said: "Fearna is a nationally significant project, capable of storing vast amounts of intermittent wind energy and releasing it back to the grid when it is needed.
"The scheme could supply over half of Scotland's homes, making it a critical component of the country's future energy system."