In a motion to a meeting of the full council on Thursday, external, the group said: “Postal household delivery in many parts of Highland is now infrequent, sometimes as little as once per week.
“The consequences can be extremely serious, including missed medical appointments, late correspondence from banks, solicitors, as well as personal letters and cards from family and friends arriving late.”
Royal Mail said its latest published figured showed that its performance was improving.
In the Inverness postcode area, which covers much of the Highland Council area, a spokesperson said first class next-day delivery increased from 78.4% in the previous quarter to 82.8% in the latest quarter.
They said: “We know how important it is that people and businesses across the Highlands receive their mail when expected, and we take concerns about delays seriously.
“Delivering mail across the Highlands involves some of the most geographically challenging routes in the UK.
“Mail continues to be delivered across the region every day, although individual routes can be affected by short-term local issues. Where this happens, our teams work to deliver any outstanding mail as quickly as possible.”
Last month, Royal Mail said it was recruiting to fill 12 vacancies at its Inverness delivery office.
Earlier this month, the Communication Workers Union (CWU) claimed the number of postal staff quitting Royal Mail had gone up since the company introduced a new delivery system earlier this year.
Royal Mail said the reforms were part of its plan to deliver a more “efficient and financially sustainable” service.