Mobile operator Three is experiencing technical problems after a planned payment systems upgrade has overrun.

The problem left customers unable to view their account details, top up, or renew monthly ‘add-ons’.

A message in the Three mobile app this morning told customers that payment functions would be unavailable as systems were upgraded. The deadline given for the work to be completed was 03:00 GMT today (11 March).

However, as this deadline has passed, the message has been replaced with the following note when users try to top up:

“Sorry, there is a slight technical glitch. If you try again now, this service should work. In the unlikely case it doesn’t, please try again later. Sorry for the inconvenience.”

A call to Three’s customer service number, 333, confirmed the problems: “We’re having some technical problems with our systems at the moment and can’t assist with account specific queries. If your phone is not lost or stolen, and you need assistance with your account, please call back later, or try our Three app for full billing info and support.”

However, customers are unable to access account details on the My3 mobile app or on the My3 website. Instead, they are met with the message: “We’re sorry, My3 is currently unavailable because we’re doing some essential maintenance work. We’ll be up and running again soon.”

A Three spokesperson said: “We are having some technical issues at the moment and are working to resolve them as quickly as possible.”

Three declined to say how many customers were affected by the glitch. Its Twitter customer service team said that it hopes to fix the problems by 1pm today.

The firm’s Twitter customer service team (@ThreeUKSupport) also said that customers who are forced to use top-up credit because they are unable to buy their usual ‘all-in-one’ text, calls and data bundles during the outage will not be refunded.