Tens of thousands of BT customers may have been without broadband and telephone services for most of the day, after a flood in Paddington knocked out one of the firm's major exchanges.
Customers in north and west London have been experiencing a loss of services, and BT said that customers in other parts of the country could also be affected. It is believed that the flood caused a fire, which has since been put out.
BT said in a statement that it is trying to fix the problem, but admitted that it did not have any hard estimates on when the fault will be resolved.
"We are currently working on restoring services to customers, but as this is a complex incident we cannot accurately predict when all services will be restored. We will issue further updates as the situation changes," the firm said.
The telecoms giant also said that customers would need to use other phones to contact the emergency services if required.
"Any customers needing to make calls to the emergency services who have a problem using their phones are advised to use their mobile phone, or a friend's or neighbour's working phone," BT said.
A number of other ISPs which have access to the exchange have also been affected, including TalkTalk.
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago