An investigation has been launched into BT following the major disruption to 999 call services on Sunday.

Emergency services across the country reported 999 calls were failing to connect because of a technical fault.

BT, which manages the 999 phone system, apologised for the problems which were resolved by Sunday evening.

The communications regulator, Ofcom, will now investigate whether BT failed to comply with its regulatory obligations.

In a statement, Ofcom said its rules required BT and other providers to take "all necessary measures to ensure uninterrupted access to emergency organisations as part of any call services offered".

"They also require providers to take all necessary measures to ensure the fullest possible availability of calls and internet in the event of catastrophic network breakdown," it added.

"Ofcom's investigation will seek to establish the facts surrounding the incident and examine whether there are reasonable grounds to believe that BT has failed to comply with its regulatory obligations."

While the incident was ongoing Cheshire Fire and Rescue Service warned of a 30-second delay to connect to 999, while Suffolk Police said its system was not working to full capacity.

The Metropolitan Police and Bedfordshire Police also reported difficulties.

The cause of the problems with the 999 service remains unclear, but the issues persisted even after BT switched to a back-up system.

The government has said it took BT nearly three hours to alert ministers to the problems it was experiencing.

A BT spokesperson said the firm was "nearing the end of a full, internal investigation and expects to share the findings with government, the emergency services and Ofcom by Thursday".

"This will examine the technical aspects of what triggered Sunday's incident, the process of moving over to the back-up system and the timings of communications to the emergency services, Ofcom and government."

Speaking after an urgent question was asked in the Lords about the incident, Labour peer Baroness Anderson of Stoke-on-Trent called it "an incredibly disconcerting event".