Three has been fined nearly £1.9 million by the regulator after the mobile provider failed to ensure customers could contact emergency services at all times.
An Ofcom investigation found Three broke rules designed to ensure everyone can contact the emergency services at all times, which the regulator said is of the utmost importance to public health and safety.
It has fined Three £1,890,000, after uncovering the weakness in the mobile operator's emergency call network.
In October 2016, Three notified Ofcom of a temporary loss of service affecting customers in Kent, Hampshire and parts of London.
Ofcom's investigation found emergency calls from customers in the affected area had to pass through a particular data centre in order to reach the emergency services - meaning Three's emergency call service was vulnerable to a single point of failure.
The regulator said Three's network should have been able to automatically divert emergency calls via back-up routes in the event of a local outage. But these back-up routes would also have failed because they were all directed through this one point.
To resolve the incident and address the underlying network weakness, Three added an extra back-up route to carry emergency call traffic.
Ofcom found Three had breached the requirement to ensure uninterrupted access to the emergency services due to the weakness identified in Three's network.
Ofcom said Three did not act deliberately or recklessly, but the fine reflects the seriousness of the breach, given the potential impact on public health and safety.
The investigation has now been settled and the penalty includes a 30% reduction to reflect the co-operation offered by Three during the investigation, including admitting the breaches identified by Ofcom.
Ofcom also acknowledged Three has taken steps to ensure ongoing compliance with its emergency call service rules.
The fine, which must be paid to Ofcom within 20 working days, will be passed on to the Treasury.
Ofcom said it expects all providers to satisfy themselves that their networks do not have any single points of failure in the routing of their emergency call traffic, which could reasonably be avoided.
Gaucho Rasmussen, Ofcom's enforcement and investigations director, said: "Telephone access to the emergency services is extremely important, because failures can have serious consequences for people's safety and wellbeing.
"Today's fine serves as a clear warning to the wider telecoms industry. Providers must take all necessary steps to ensure uninterrupted access to emergency services."
Three said in a statement: "Providing our customers with uninterrupted access to emergency services is a requirement we take extremely seriously.
"Three therefore acknowledges Ofcom's decision today to fine Three for a single point of vulnerability on Three's network. However, this vulnerability has not had any impact on our customers and only relates to a potential point of failure in Three's network...
"Ofcom has also recognised today that Three has cooperated fully during its investigation and has taken steps to further strengthen the resilience of Three's network."