Hundreds of Post Office and Royal Mail users have reported outages with the service, including at Stroud Post Office, Minchinhampton Post Office and the Post Office in the Co-Op, Stonehouse.
According to DownDetector, there have been issues reported since lunchtime, with more than 600 reports since 1.30pm.
A Post Office spokesperson said: “We are aware that there is a major outage across the Post Office estate.
“We sincerely apologise for any inconvenience caused. We are working hard to restore service as soon as possible.
“This is not a cyber-related issue.
“We are aware of what has caused this outage and the earliest that Post Office services will resume is at 4pm today.”
Stroud Post Office has since reopened.