BT has said some of its customers are experiencing problems with its broadband and telephone services.
"Sorry, we're aware of a problem & working to fix ASAP," it tweeted shortly after 0900 BST.
The firm's Plusnet broadband division has also said that it is facing issues.
BT said that "power issues at one of our internet peering partners' sites in London" were the cause. "Engineers are working to fix things as fast as possible," it added.
A spokeswoman was unable to provide more information.
However a statement by Plusnet added: "Although the symptoms are similar to the problems reported over the last few days, upon investigation we believe this issue to be unrelated."
BT's service status pages indicate that subscribers in parts of England, Wales, Scotland and Northern Ireland have all been affected.
Downdetector - a site people use to report internet-related faults - indicates that it had received hundreds of complaints about BT's service more than an hour before the telecoms firm tweeted about its issue.
The problem comes a day after BT's Openreach internet infrastructure business was criticised by MPs over its alleged failure to "adequately" invest in fast fibre.
The chief executive of TalkTalk, a rival telecoms firm, has reiterated her call for the unit to be split off and run as a separate business.
"My business spends hundreds of millions of pounds a year with BT - my single largest competitor, also my single largest supplier," Dido Harding told the BBC's Today programme.
"The reason why I care about this is that my customers are fed up with the fact that the network doesn't work well enough and they know that they're not getting the service they deserve."
BT said on Tuesday that it had invested more than £1bn a year in infrastructure and was improving service levels.
The communications regulator Ofcom is currently considering the matter and is due to make a decision on whether Openreach should be spun off soon.
Shares in BT fell more than 1% in early trade.