People in rural areas are more likely than their urban counterparts to depend on their local post office for key services, says a report by the charity.
These include banking, collecting state benefits, pensions and purchasing household goods – as well as postal services.
As the number of pubs, banks and shops decline, Citizens Advice is calling on government to commit to continued funding of these community institutions.
The government's decision on funding the postal network, including important rural services, has not yet been confirmed following a public consultation from December 2016.
Citizens Advice is the statutory watchdog for postal consumers.
It wants government to publish its consultation response and commit appropriate funding for the next three years to ensure rural post offices remain open.
A poll of over 800 rural residents found a over a quarter of people rely on their local post office for learning about local events and services.
The survey also found 1 in 5 people said they would lose contact with neighbours or friends if it wasn't for their local post office.
This was particularly important for people over 65 years old, people with a disability, carers and those who are digitally excluded.
Small businesses also rely on post offices more in rural areas.
Some 7 out of 10 small rural businesses anticipate they would have higher costs without their post office.
An estimated 8 out of 10 rural businesses says it would take more time.
Citizens Advice chief executive Gillian Guy said: "With pubs and banks closing up shop in many rural areas, post offices remain crucial community institutions.
"This is especially true for vulnerable groups like older people, people with a disability, carers and those who do not have access or the skills to use the internet.
"That is why we're calling on the government to ensure vulnerable groups are protected by confirming funding for the postal network, including crucial rural post offices."
Citizens Advice provides policy advice and strategic projects that contribute the operation of the postal market.
It also works to ensure consumer needs are represented in decisions on the future of the post office network.
The Citizens Advice report on Rural Post Office Use is available here.
Citizens Advice service staff are supported by more than 21,000 trained volunteers, working at over 2,500 service outlets across England and Wales.
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