BT and Plusnet users have been unable to access some websites again, after problems with the service yesterday.
The firm said the fault stemmed from Telehouse North, an exchange in London's Docklands.
Telehouse North confirmed that its network had been affected by a tripped circuit breaker but that engineers were now on site.
There were reports of other organisations experiencing connectivity trouble, including HMRC.
On Wednesday, many customers found they were unable to visit certain websites and access some internet banking services.
Those difficulties were 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.
Telehouse North said the tripped circuit breaker at its facility had impacted "a specific and limited group of customers".
"The problem has been investigated and the solution identified," a spokesman added. "Our engineers are working with our customers on the resolution right now."
