BT broadband users have been unable to access some websites again, following problems with the service yesterday.
The firm said the fault stemmed from a power failure at another "connection partner" in London's Docklands.
Yesterday many customers found they were unable to visit certain websites and access some internet banking services, which was blamed on a separate power cut at the Equinix Telecity Harbour Exchange.
A spokesman for Linx - a partner of Equinix - confirmed that its systems were unaffected today.
The UK's biggest internet provider said the latest problem was affecting less than 5% of customers' internet access and engineers were working on the affected site.
The company said: "We're sorry that some BT and Plusnet customers are having problems connecting to some internet services this morning. This is due to Telehouse North, one of our internet connection partners in the Docklands, suffering a substantial power failure. This is affecting BT and other providers.
"We are redirecting traffic to reduce the impact on customers and the issue is now affecting less than 5% of customers' internet usage.
"Engineers are on site and we hope will be able to fix the problem shortly."
There have been renewed reports of some users being unable to access certain internet banking services.
Sorry if you're unable to access some websites this morning on BT Broadband, we're investigating the cause and will update you all soon
— BTCare (@BTCare) July 21, 2016
We’re sorry that some BT customers can't connect to some internet services today.We're working to reduce impact, engineers are on site now
— BTCare (@BTCare) July 21, 2016
MPs warned this week that BT must "put its house in order" or face a break-up after failing to invest in its Openreach network arm potentially to the tune of hundreds of millions of pounds a year.
Consumer group Which? called for compensation for those affected.
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