Flooding at a BT exchange affected broadband and telephone services today.
Tens of thousands of customers in parts of north and west London and possibly others across the country might have been hit following the incident in the Paddington area of west London, a BT spokesman said.
"Following major flooding at a BT exchange in the Paddington area, tens of thousands of customers in parts of north and west London may be experiencing a loss of broadband and or telephone service," a statement said.
"Customers in other parts of the country may also be affected.
"We are currently working on restoring services to customers. However, 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.
"Any customers needing to make calls to the emergency services who have a problem using their phones are advised to do so by using their mobile phone, or alternatively by using a friend or neighbour's working phone."
A spokesman for London Fire Brigade said they were called at around 7.30am to a "flooding incident" in an electrical intake room in the basement of a building.
A Barclays spokesman said three central London branches - Marylebone, Portman Square and Sloane Square - had been closed all day following the incident.
He said: "The branch network relies on telephony links to be able to operate and without connection to the telephone network there was no alternative but to close and redistribute staff to other branches."