Councillor Moraig Lyall has stood down from the council’s planning committee.
The Shetland Central member says she believes councillors have been “relegated” to a “postscript in the decision-making process”.
She says the function of the committee – on most occasions – appears simply to confirm that planning officials “read the rule book”.
The decision comes after planning approved Scalloway’s new Co-op on Monday.
At that time, Mrs Lyall warned she could not support the application because of serious concerns she had about the proposals.
Now, in a statement Mrs Lyall says there is “little ability” for the committee to explore relevant issues “in a Shetland context”.
She said: “Following the planning committee, I sent the chair my resignation from the group.
“As it stands, it seems to me that there is little ability for the committee to explore what might or might not be the relevant issues in a Shetland context.
“The function of the committee on most occasions is simply to confirm that the planning officer ‘read the rule book’ correctly.”
She added: “This is not a reaction to yesterday’s meeting, but the conclusion of a process of coming to understand that the committee is tightly curtailed in its deliberations.
“It has met only three or four times annually for the last few years which some might argue is evidence of a system which is working well, as so little is brought to the committee.
“But I see it as having relegated the councillors to a postscript in the decision making process.”
Mrs Lyall’s decision comes a year after Shetland West councillor Catherine Hughson also resigned from the planning committee.
Mrs Lyall added: “I don’t know if they’ll try to replace me at this late stage in the council, but I doubt there would be a queue at the door if they did, as they were unable to replace councillor Hughson when she resigned.”