A British Airways computer worker has accused the company of ignoring warnings about outsourcing work and cutting jobs over the past year.
Speaking after an IT shutdown left 75,000 bank holiday travellers stranded, he said Heathrow had been hit by power outages in the past, but BA's system was always resilient enough to be protected.
The GMB union called on BA to halt any further job cuts and to bring IT work back in-house from India.
Bill Francis, Head of Group IT at BA's owner International Airlines Group (IAG), has sent an email to staff saying an investigation so far had found that an Uninterruptible Power Supply to a core data centre at Heathrow was over-ridden on Saturday morning.
He said: "This resulted in the total immediate loss of power to the facility, bypassing the backup generators and batteries. This in turn meant that the controlled contingency migration to other facilities could not be applied.
"After a few minutes of this shutdown of power, it was turned back on in an unplanned and uncontrolled fashion, which created physical damage to the system, and significantly exacerbated the problem.
"This was entirely a problem relating to the power supply. It was not an IT failure, and there were no software issues.
"The fix consisted of physically replacing servers that had been damaged, then bringing all of BA's 700-plus applications back online in a controlled fashion while ensuring that all data was consistent across the system. All of the systems are now back up and running."
The BA worker, who did not want to be named, told the Press Association that 600 IT jobs had been lost since March last year, with work being outsourced to India.
The worker said: "We have been warning that to rip out the knowledge and experience from what is a very complex IT estate would have serious consequences in terms of long-term maintenance of the system as well as any recovery from any hiccups.
"We had built up a tremendous reputation for excellence, with instantaneous recovery from any problems.
"The system was always meant to be resilient and protected from any power outage."
Mick Rix, national officer of the GMB, said: "BA's IT workers keep the business going. Why would IAG want to hamper the success of BA by getting rid of valuable IT personnel?
"We are calling on IAG to stop any further redundancies and offshoring of work currently done by BA IT staff.
"We are also urging them to bring work back in-house."
Willie Walsh, chief executive of IAG, has apologised to BA's customers affected by the weekend of chaos and praised airline staff and chief executive Alex Cruz for the way they handled the fiasco.
Mr Walsh said the cause of the problem had been identified and that efforts were being made to appease customers.
He told the BBC: "I'm pleased that British Airways has been able to recover from the significant disruption that they faced on Saturday.
"I think the team at British Airways, under the leadership of Alex Cruz, has done everything possible to get British Airways back flying a full schedule as quickly as possible. We clearly apologise to any of our customers who were disrupted.
"We know the cause of the problem - it was not an IT failure, it was a problem caused by the failure of electrical power to our IT systems.
"We understand what happened, we're still investigating why it happened and that investigation will take some time.
Mystery surrounds the cause of the power surge, with National Grid and local energy providers saying there had been no supply issues on Saturday.
The cause of the initial power outage and the subsequent surge has not yet been revealed.
BA said: "We would never compromise the integrity and security of our IT systems. IT services are now provided globally by a range of suppliers and this is very common practice across all industries and the UK Government.
"The incident on Saturday was not an IT issue, it was a power issue. There was a total loss of power at the data centre. The power then returned in an uncontrolled way, causing physical damage to the IT servers. It had nothing to do with outsourcing of IT.
"We are undertaking an exhaustive investigation to find out the exact circumstances and most importantly ensure that this can never happen again."