A flood in a BT exchange in the Paddington area has knocked out telephone and broadband services to tens of thousands of customers in north and west London. Customers in other parts of the country may also be affected, the network operator said.
BT said in a statement that this was a "complex incident".
"We cannot accurately predict when all services will be restored. We will issue further updates as the situation changes," it said.
BT advised customers who need to call the emergency services to use their mobile phones or a friend or neighbour's working phone.
"We are currently assessing the impact of the damage but will only know the full extent of the impact once the site has been declared safe and our engineers are able to enter the building," BT said.
The network failure comes just months after "White Noise", a simulated countrywide network failure exercise, details of which are still under wraps.
A BT spokesman said the present failure was a different order of magnitude. "We are using our normal emergency procedures," he said.