Mobile provider O2 is investigating why phone users in London suffered a signal outage that prompted a government minister to tell the company to “sort it out”.
Customers lost reception for up to five hours on Monday, upsetting many including business secretary Sajid Javid.
Normal service was restored on Monday night.
In a statement today O2 said: "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.
Annoyed customers took to social media to vent their frustration on Monday.
Mr Javid tweeted: "No signal, O2. Please sort it out."
Emmerdale star Michelle Hardwick said: "Come on O2, I want my signal back please!"
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