Millions of Three UK mobile phone network customers were unable to use their phones after a network meltdown.

The company, which has 10 million users in the UK, said the the problem started on Wednesday night and service had started to be returned to some customers on Thursday. The network advised customers to switch their phones off and on again to restore service.

Three was unable to say how many customers had been affected or whether they would receive compensation, but admitted that a “significant majority” of users had initially been left without a phone signal or access to data. It said a minority were still affected by Thursday afternoon.

The problems started on Wednesday at about 11.30pm after repairs to Three’s 3G network infrastructure.

“Following technical difficulties with our services across voice, text and data, some customers will be experiencing an intermittent service,” a Three spokesperson said. “Our engineers are working to fix the issue and the service is returning to normal, and we expect it to be resolved over the course of the day.

“We advise our customers to turn their phones off and on or turn airplane mode on and off, which may resolve the issue. We are sorry for the inconvenience caused to our customers.”

Downdetector, a website that tracks outages across various networks, said it had received reports of problems from thousands of Three users in Manchester, London, Glasgow, Birmingham, Leeds, Liverpool, Nottingham, Edinburgh, Bradford and Sheffield.

Ofcom, the industry regulator, said it was too soon to say if it would carry out an investigation into the outage. A spokesperson said: “We are aware of the problems experienced by some Three customers. We are speaking to Three to find out what is happening.”

The Three website also went down for a number of hours and later carried the message: “ We’re aware of network issues and are working hard to fix them. We know the impact this is having on your day, and we’re sorry. Rest assured, we’ve got our best minds on the job.”

Many Three mobile users complained on Twitter they had been unable to make calls or go online for hours and that there had been no communication from the company.

After a long silence, Three tweeted on Thursday morning:

Adam French, a consumer rights expert at Which?, said: “Huge numbers of Three customers have woken up to find they are unable to make calls or use roaming data and will be left feeling angry and frustrated as a result. The mobile operator has to do right by its customers by keeping them informed, ensuring no one is left out of pocket, and we expect those affected by this lengthy lack of service to be compensated.”

He added: “The UK is already lagging behind the rest of the world on the provision of 4G. Failures like this will only intensify the pressure on the government, industry and the regulator to up their game and ensure we get a decent service.”

Last December rival mobile phone provider O2 promised its 25 million UK customers that they would receive up to two days’ credit in compensation as a goodwill gesture after a UK-wide outage left them unable to get online.

Ofcom says on its website: “Depending on the circumstances, it may be appropriate for your provider to offer you some money back while repairs are being carried out.” If this takes much longer – for example, if it takes longer than usual to access a mast site to undertake repairs – customers could receive an additional refund or account credit.

It adds that customers who have been without service for some time may also have the right to leave their contracts without penalty and should check the terms of their contracts.

Ofcom advises consumers to complain to the company directly, but if the complaint is still unresolved after eight weeks, they can submit it to an independent alternative dispute resolution scheme.

Which? said: “If you were affected by the Three outage, you can make a consequential loss claim if you can show you incurred reasonable losses and the cause of the outage was a failure to provide the service with reasonable care and skill. You can do this if you are a consumer – for example, if you incurred expenses in order to use internet services for personal reasons.”

Businesses may also be able to make a claim if they suffered losses – although not under the Consumer Rights Act – and are advised to check what level of coverage is promised in their contracts.