Power construction engineer David Askew, 52, from Braintree, Essex, suffered fatal head injuries after falling from a wooden ladder at London's Canonbury Telephone Exchange on 27 October 2006.
British Telecommunications Plc (BT) was prosecuted after an investigation by the Health & Safety Executive (HSE).
Southwark Crown Court heard Mr Askew was installing distribution boards and running cabling as part of his work and would have been working at a height of more than four metres. He fell from a nine-step wooden ladder, sustaining a serious head injury and died 18 days later.
The HSE investigation found a number of issues including a failure to ensure the work at height was properly planned, and that Mr Askew was provided with suitable access equipment for work at height.
Two wooden ladders found at the scene had not been subject to an annual inspection, contrary to BT's own health and safety policy.
BT, based in Newgate Street, London, was found guilty of breaching section 2(1) of the Health and Safety at Work etc. Act 1974 at a previous hearing. Today, they were fined £300,000 and were ordered to pay costs of £196,150
HSE inspector Nicola Maisuria said: "The fact that this incident was entirely avoidable makes Mr Askew's death all the more tragic. The dangers posed by work at height are well known, yet BT failed to create the conditions to ensure this task was carried out safely and the appropriate access equipment was used. Employers have a responsibility to ensure that work at height is properly planned and organised."
Falls from height remain the most common cause of workplace fatality. In 2008/09 there were 35 fatalities, 4,654 major injuries and a further 7,065 injuries that caused the injured person to be off work for over three days or more, due to a fall from height.