Mobile operator Three has apologised as more than 10,000 customers experienced outages after problems with its service over the last few days.

The company, which has more than 10 million UK customers, issued apologies in posts on X on Saturday, Sunday and Monday regarding a string of problems affecting voice calls and the ability to use mobile data.

“We are very sorry for the issues with service over the past few days and sincerely apologise for any inconvenience caused,” Three said in a post on Monday. “We know a number of our customers are still waiting on their service to be restored, and our engineers are working to fully fix it.”

Downdetector, which tracks outages, shows that more than 12,000 people have reported that their phone services have suffered some form of fault.

A spokesperson for Three UK said the issue that affected customers over the weekend is separate to the problems that emerged on Monday, but the end result is the same type of disruption to the network for mobile phone users.

“Following an issue with our network that started earlier, services are now recovering,” it said in an update on X.

Last month, the Competition and Markets Authority (CMA), the UK regulator, launched an investigation into the proposed merger of Vodafone and Three, which would create the UK’s largest mobile phone operator.

The firms said the deal would result in an additional investment of £11bn in the UK.

In 2016, the CMA and the European Commission blocked Three’s attempted takeover of O2, arguing that it would have risked higher prices.

However, in 2022 Ofcom, the UK telecoms regulator, changed its long-held stance, saying it was now more open to consolidation in the sector. Ofcom had previously argued that a reduction to only three networks could harm UK consumers.

In a separate move the government is also investigating the stake in Vodafone held by a United Arab Emirates-backed telecoms group.

The Cabinet Office warned that the 14.6% stake held by Emirates Telecommunications Group, which is also known as e&, amounted to a security concern given Vodafone’s strategic role in UK telecoms services.

The government has now ordered a “national security committee” to be set up at Vodafone that will oversee and monitor any sensitive work the telecoms firm carries out that could have an impact on national security.

Vodafone UK has a number of government contracts, and Three UK is owned by the Hong Kong-based CK Hutchison, which may raise concerns about foreign joint-ownership of a key national asset, particularly given the city’s status as part of China.

Vodafone UK has public sector contracts with the Ministry of Defence, the Ministry of Justice, NHS 111 and local police forces.