The Kilcreggan resident, who was elected as a Conservative councillor in 2017, has stood down for personal reasons.
She had been chairperson of the council’s Helensburgh and Lomond Area Committee since October 2020, following the passing of Helensburgh and Lomond South Councillor Ellen Morton.
Arrangements for the next steps are to be reported to the next ordinary meeting of the full council, which is scheduled for Thursday November 25.
A virtual full council meeting is taking place on Thursday October 28, but is expected to solely discuss responses to consultations by the Scottish Government.
It is not known whether a by-election will take place before the scheduled council elections on Thursday May 5, 2022, or whether the Lomond North ward will continue with its two remaining councillors – George Freeman and Iain S Paterson – until then.
Ms Morgan was also a member of the council’s community services committee, and had been present at a virtual meeting of the full council on Thursday September 30.
A spokesperson for the council said: 'Councillor Barbara Morgan has resigned from the council, for personal reasons, with immediate effect.
'In line with procedure, we are making arrangements for her resignation and next steps to be reported to the next ordinary meeting of full council.
'While the post remains vacant, Lomond North (Ward 9) will continue to be served by councillors Iain Shonny Paterson and George Freeman.'
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