Thousands of passengers of the beleaguered British Airways faced a third day of disruption at Heathrow on Monday after a global computer crash that unions blamed on outsourcing of IT services to India that grounded planes, causing millions of pounds to the airline.

The airline is "closer to full operational capacity" after an IT power cut resulted in mass flight cancellations at Heathrow and Gatwick.

Thousands of passengers remain displaced, with large numbers sleeping overnight in terminals.

The British Airways has not explained the cause of the power problem. So far on Monday, 13 short-haul flights at Heathrow, Europe's busiest, have been cancelled.

Some British media reports today suggested that BA could be hit with a bill for compensation costs of more than 100 million.

The BA is liable to reimburse thousands of passengers for refreshments and hotel expenses, and travel industry commentators have suggested the cost to the company – part of Europe's largest airline group IAG – could run into tens of millions of pounds.

Customers displaced by flight cancellations can claim up to 200 pounds a day for a room, 50 pounds for transport between the hotel and airport, and 25 pounds a day per adult for meals and refreshments, the BBC reported.

Heathrow advised affected BA passengers not to travel to the airport unless their flights had been rebooked, or were scheduled to take off on Monday.

Passengers on cancelled flights have been told to use the BA website to rebook.

Chief executive Alex Cruz has posted videos on Twitter apologising for what he called a "horrible time for passengers".

But no-one from the airline has been made available to answer questions about the system crash, and it has not explained why there was no back-up system in place, the BBC reported.

Heathrow advised affected BA passengers not to travel to the airport unless their flights had been rebooked, or were scheduled to take off on Monday.

The airline has claimed it was making "good progress" in recovering from the worldwide IT glitch.

Thousands of passengers had their travel plans disrupted as a result of the problem, with one workers' union blaming the outsourcing of IT jobs to India for the chaos, a claim denied by the airline.

BA's GMB union has said outsourcing IT jobs to India could have made the problems worse.

The union spokesperson said it could have been avoided had "hundreds of dedicated and loyal" not been replaced by cheaper Indian staff in 2016.

The Sun newspaper, quoting one source close to the airline said the problems could have been limited had IT staff outsourced to India known how to get its back-up system online quickly.

The source said the system "failed to take over when the primary (IT system) failed due to a power cut." 

They added: "The third-party support providers are generally quite inexperienced meaning it makes a situation like this difficult to recover." 

Meanwhile, the BA said there was no evidence the failure was the result of a cyber attack.

Experts predict the knock-on effect on the BA could continue for several days.

Passengers faced hours-long lines to check in, reclaim lost luggage or rebook flights at Terminal 5, BA's hub at Heathrow.

BA operates hundreds of flights from Heathrow and Gatwick on a typical day and both are major hubs for worldwide travel.