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.
Electronics and computer chain the latest high street retailer to fall into difficulties
Incisive Media and Investec Asset Management supported fundraiser crosses Atlantic in 40 days
Alphabet's health sciences division Verily have been messing with AI algorithms
North Korea's cyber attack capabilities are expanding fast - and turning their fire on a wider range of targets