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 fault is also causing several banks to be unable to offer online services.
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.
Some internet banking customers at Lloyds, Barclays, Natwest, Halifax, Santander, HSBC and Royal Bank of Scotland have been unable to log into their accounts.
Lloyds said that this was caused by a fault with an internet service provider.
It did not name BT, but the BBC understands from speaking with others in the industry that this is the case.
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 – indicated that it had received hundreds of complaints about BT’s service more than an hour before the telecoms firm tweeted about its issue.
It also recorded a spike in reports from bank customers at about 0830 BST.
BT’s Plusnet broadband division has also said that it is facing issues.
“Although the symptoms are similar to the problems reported over the last few days, upon investigation we believe this issue to be unrelated,” it said on its site.
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.