The IT outage that grounded all British Airways flights over the bank holiday weekend and could cost the airline £100m in compensation is thought to had been triggered by a human mistake, rather than by equipment failure.

Human error is expected to be the main focus of the investigation into the incident, which disrupted the travel plans of approximately 75,000 passengers last week.

The failure is understood to be related to an uninterruptible power supply (UPS), which is tasked with delivering supply from the mains and can fall back onto a battery back up and a diesel generator in case of emergency.

Earlier this week, BA's parent company IAG admitted that power was momentarily lost at Boadicea House, a data centre. However, the investigation is understood to have found the UPS, supplied by Hitec Power Protection, was working as normal at the time of the incident.

According to The Times, a source from within the airline has suggested a contractor carrying out maintenance work might have inadvertently switched the power off.

While the report remains unconfirmed, an email from IAG's head of group IT Bill Francis appears to substantiate this version of events.

"This resulted in the total immediate loss of power to the facility, bypassing the backup generators and batteries," reads the email leaked to the Press Association.

"It was turned back on in an unplanned and uncontrolled fashion, which created physical damage to the system."

All of the airline flights at London Heathrow and Gatwick were left grounded, after the computer system used by BA suffered a "major" global failure on Saturday (27 May). By late afternoon on the same day, the carrier's computer systems, app and website had all gone into shut down, leading to chaotic scenes at both airports as passengers sought explanations.

BA chief executive Alex Cruz blamed the IT failure on a power surge and promised that the airline would compensate the passengers, but denied cost-cutting was the cause, despite suggestions some 600 IT jobs had been lost since March last year.