Three has apologised after more than ten thousand people in the UK reported they were left without mobile service.
Downdetector, which tracks outages, showed over 12,000 people reported their services, including calling and using mobile data, were not working.
It is the latest outage to hit Three, after it apologised on both Saturday and Sunday over similar incidents.
Three said it was “very sorry for the issues with service over the past few days”.
“Following an issue with our network that started earlier, services are recovering,” a spokesperson said in a statement.
“We appreciate a number of our customers are still waiting on their service to be restored and our engineers are working to fully fix it.
“Our customer service channels continue to be unavailable, so please check our social feeds for the latest updates.”
Three has around 10.5m customers across the UK, according to its website, but it is unclear how many of them are affected by the outages as those who rely on mobile internet may be unable to report the issue.
Reports began over the latest incident at 1000 GMT on 12 February, with frustrated customers taking to social media to voice their concerns – with some even threatening to leave the network altogether.
There have also been reports of the outages affecting some people using smaller providers which use Three’s network, as there have been 800 reports from customers of one such firm – Smarty – according to Downdetector.
It is unclear whether customers will be able to claim compensation for the outage, although according to the website of telecoms regulator Ofcom, it “may be appropriate” for providers to offer refunds “while repairs are being made”.
The outages come two weeks after the UK’s competition watchdog announced it was investigating a proposed merger between Three and Vodafone.
The Competition and Markets Authority (CMA) will look into whether the deal could harm consumers by leading to reduced choice or higher prices – as it would create the UK’s biggest mobile network.
The firms said the deal would result in an additional investment of £11bn in the UK.