The Government is being urged to continue funding rural post offices amid the continued closure of pubs and banks.
Research for Citizens Advice found that one in four people living in villages or other rural areas relied on their local post office for information about local services or events.
A survey of 800 rural residents also showed that one in five believed they would lose contact with neighbours of friends if their local post office closed.
Small business feared higher costs without a local post office, said 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."
A Government spokesman said: "Between 2010 and 2018 the Government will have invested nearly £2 billion in modernising and maintaining what is now the most stable post office network in decades, with more than 11,600 branches across the network and 4,400 open on Sundays.
"The Government remains committed to funding the Post Office in order to further enhance the national network. We will publish our response to the consultation in due course."