People with affected phone lines will be unable to make emergency calls, warns BT.
A major flood at a BT exchange in Paddington, London has affected broadband and telephone services across the UK.
In a statement BT said it could not predict when either service would be restored.
"Tens of thousands" of customers have been affected, said the firm, with the majority in north and west London.
BT confirmed that some mobile phone services may be affected by the incident, which also caused a fire.
London Fire Brigade attended the scene at 7.30am on 31 March. The flood was caused by an electric fault.
The BBC was alerted to the fault by IT consultant Jerry Sanders, who said customers as far afield as Potters Bar, Hertfordshire and Nottingham were reporting problems with Pipex UK broadband coverage.
Pipex UK's parent company TalkTalk said that its service had been affected by the incident.
Telecoms company Gradwell uses Carphone Warehouse (also part of TalkTalk) networks and equipment to provide customers with its business telecoms services.
It has published a list of 437 exchanges across the UK it says have been affected.
Managing Director Dr Peter Gradwell told the BBC that some of the company's 45,000 Voice over Internet Protocol (VoIP) telephone customers had also reported problems.
"A lot of traffic has been re-routed," he said. "There's a lot of work going on. I would hazard a guess that everything will be back to normal on Tuesday (after the Easter break)".