The mailing featured a wind turbine and plans detailing the development of a wind farm and implications for the local economy and environment.
Recipients of the mailing included the anti-wind-farm pressure group Sustainable Shetland and its members.
They did not believe claims including 50 per cent of the profits would stay with the Shetland community and upwards of £18 million profits to the Shetland Charitable Trust.
They also felt claims about the development's carbon footprint and the Shetland economy were misleading.
Viking Energy said 45 per cent of the wind farm project was owned by Shetland Charitable Trust, which it saw as a common good fund to benefit the wider Shetland community.
It had believed 50 per cent of the profits would be paid to the people in Shetland rather than companies. In hindsight, it agreed the leaflet should have defined community more clearly.
The company further said its figures showing estimated financial benefits were subsequently found to be underestimates.
But the ASA considered the claim relating to 50 per cent of profits staying with the Shetland community was misleading.
It also noted the company had not provided adequate documentation to show a price for the electricity that would be generated by the wind farm and considered this was because at the time the ad published it was not possible to know the price.
Therefore, the claims relating to money injected into the town’s economy and profits made by the charity were misleading.
The watchdog concluded the ad must not appear again in its current form.