Trevor Maloney, 32, from Tottenham, suffered burns to both hands and his face in the incident on 24 April 2012.

He was hospitalised for four days, but has since made a full recovery.

The ground worker was unaware of the presence of the cable as he used an electric breaker to remove paving stone mortar.

SIAC had been contracted by Westminster City Council to redevelop Leicester Square – a major project that included substantial excavation and ground work to resurface the main terraces and side roads.

Westminster Magistrates’ Court heard that due to the large number of underground services in the vicinity, the local authority gave clear instruction that any shallow cables or pipework in areas to be concreted were to be protected by sand and steel plating.

There was also a further requirement to put marker tape on plating covering electricity services so they could be easily identified in the event of future work.

But during the excavation of old paving outside the Hippodrome casino, in Cranbourn Street, an electricity cable was discovered that was so shallow the preferred protective method could not be used.

A work-around was agreed between SIAC and the council, but when the new paving was completed in January 2012 it was unclear whether it had been done.

In March 2012, part of the newly installed paving had to be dug up for underground repair work unconnected to the renovation project. SIAC was not involved in this work, but was later asked to reinstate the paving.

Maloney was one of two workers tasked with removing old mortar so that the slabs could be re-laid, but just a few minutes after starting the work he struck the shallow cable.

It short-circuited and created an explosion and flames.

HSE found that the cable was partially above the concrete foundation and concealed within the mortar, unprotected by any steel plating.

Magistrates were told that had it been adequately protected, or its location properly identified to the ground workers as an area in which to take extreme care, then the incident could have been avoided.

SIAC Construction Ltd, of Dublin, was fined £20,000 and ordered to pay £5,002 in costs after pleading guilty to a  safety breach.

After sentencing HSE Inspector Loraine Charles said: “It is essential that when any work that might disturb live underground services is being carried out, all practicable steps are taken to determine the location of those services, and to arrange the work so that the risk of damaging any service is minimised.”