BT has been fined £42 million for delays in installing high-speed lines.

Ofcom says it has imposed the penalty after BT cut its compensation payments to other telecoms providers for late installations.

BT has admitted failings, and agreed to compensate the companies affected.

The penalty is a result of an investigation by Ofcom into BT's network arm, Openreach.

The investigation found that, between January 2013 and December 2014, BT misused the terms of its contracts to reduce compensation payments owed to other telecoms providers for failing to deliver 'Ethernet' services on time.

Ethernet services are the most common type of 'leased lines' - dedicated, high-speed cables used by large businesses, and mobile and broadband providers, to transmit data. These lines also provide vital, high-capacity links for hospitals, schools and libraries.

Ofcom has taken enforcement action because BT breached rules that address the company's 'significant market power'. This market power comes from the fact that most telecoms companies rely on access to BT's network to provide services such as broadband to their customers.

Gaucho Rasmussen, Ofcom's Investigations Director, said: "These high-speed lines are a vital part of this country's digital backbone. Millions of people rely on BT's network for the phone and broadband services they use every day.

"We found BT broke our rules by failing to pay other telecoms companies proper compensation when these services were not provided on time. The size of our fine reflects how important these rules are to protect competition and, ultimately, consumers and businesses. Our message is clear - we will not tolerate this sort of behaviour."

Clive Selley, Openreach CEO, said: "We apologise wholeheartedly for the mistakes Openreach made in the past when processing orders for a number of high-speed business connections.

"This shouldn't have happened and we fully accept Ofcom's findings.

"Since I became CEO of Openreach in February 2016, we have monitored this area very closely, we have made improvements to how we process and deliver such connections, and we will make sure the same mistakes aren't repeated in future.

"This issue is unrepresentative of the vast majority of work conducted by Openreach and we are committed to delivering outstanding service for our customers."