Business Secretary Sajid Javid has urged phone provider O2 to "sort it out" after a flood of complaints from people who had lost the signal on their mobile telephones.
He tweeted "No signal @O2. Please sort it out" as annoyed customers went online to claim they have had no service for up to five hours.
People in Manchester, Bristol, Birmingham, Glasgow and parts of Northern Ireland and London have reported problems.
The phone provider, which had suggested that people reboot their handsets and turn their phones on and off again, said there were aware of "some issues" and "isolated instances".
In a series of tweets O2 told customers: "We're working flat out (and upright too) to get these fixed. Sorry for the problems" and "Sorry for all the effort you've had to put in Ems. We're making sure our engineers put in even more to fix the problems!"
O2 also insisted that "our engineers are beavering away and working to fix ASAP."
O2 tweeted: "We're aware of isolated instances where some customers have intermittent access to our service. We're investigating and will update ASAP".
An O2 spokesman said: "Further to our previous message where some O2 customers were experiencing intermittent network service, we have taken steps to minimise the impact and some customers have seen their service return.
"We apologise for any inconvenience and continue to do all we can to restore service for customers who remain impacted."