Phone provider O2 said it is launching a "full investigation" after people across the country lost signal to their mobile phones.
Business Secretary Sajid Javid was among the disgruntled customers who lost reception yesterday - some for up to five hours - and told the company to "sort it out" on Twitter.
Signal problems were reported in Manchester, Bristol, Birmingham, Glasgow and parts of Northern Ireland and London.
Normal service was restored shortly before midnight last night and the provider said it would be trying to identify the source of the disruption.
It said in a statement: "Further to our previous messages where some customers experienced some problems accessing our network, we can confirm that service returned to normal at 23.35 yesterday.
"We apologise for any inconvenience caused to those customers affected and we will now begin a full investigation to identify the root cause."
Customers were advised to switch their phones on and off if they were still experiencing difficulties, as O2 apologised to people on Twitter.
It tweeted one person: "Sorry Graham, all services should now be back up and running. A reboot should help."
Mr Javid tweeted: "No signal, O2. Please sort it out."
Emmerdale star Michelle Hardwick said: "Thanks everyone! Come on O2, I want my signal back please!"
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."
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