British Airways has returned to operating a full schedule after an IT failure caused chaos over the bank holiday weekend.
The airline said its systems are "back up and running" but accepted that work to reunite passengers with their bags is "continuing".
Experts predict the knock-on effect of the IT outage could continue for several days and BA is facing huge compensation costs, with reports suggesting the bill could top £100 million.
The airline's parent company, IAG, saw shares fall by around 3% in the first day of trading in London after the problem emerged.
Shares in IAG, which is also listed in Madrid, had already tumbled heavily in trading in Spain on Monday, wiping around 410 million euro (£357 million) off the stock.
George Salmon, equity analyst at Hargreaves Lansdown, said that while the costs of passenger compensation and refunds "could well run into the tens of millions", the episode has "undeniably put a dent" in BA's reputation for delivering a premium service.
"The worry for shareholders is that this unquantifiable impact could have longer-term consequences," he warned.
BA chief executive Alex Cruz announced that a full investigation would take place into the failure which affected 75,000 passengers as thousands of flights were cancelled.
He said: "On Saturday morning we did have a power surge. It did affect our communications systems.
"We are now focusing on making sure everyone's needs are addressed. We will make an in-depth investigation to make sure we get to the bottom of exactly why this happened and we will react.
"Absolutely this will not happen again at British Airways."
BA has said there is no evidence it was the victim of a cyber attack.
On Saturday night, travellers spent the night sleeping on yoga mats spread on terminal floors after BA cancelled all flights leaving the London hubs, while disruption continued into Sunday with dozens more services from Heathrow axed.
The IT outage had a knock-on effect on BA services around the world, while passengers who did get moving on the limited number of flights to take off from the UK reported arriving at their destinations without their luggage.
Mr Cruz said the outsourcing of jobs was not to blame for the "catastrophic" IT failure.
BA was accused of greed after the GMB union suggested the disruption could have been prevented if the beleaguered airline had not cut "hundreds of dedicated and loyal" IT staff and contracted the work to India in 2016.
Low-cost carrier Ryanair described the disruption as a "disaster" for BA and poked fun at the airline on Twitter.
Ryanair's chief marketing officer, Kenny Jacobs, said: "It was a disaster for them at the weekend and we had a but of fun on social media.
"We don't take social media seriously but we do take IT very seriously and that is why we've never had an outage."
Mr Jacobs agreed with a suggestion that Ryanair's system is less complex than BA's because it is in-house and based in Europe.
"The footprint that we have is easier to control as a result," he said.
Ryanair reported a surge in bookings for last-minute travel over the weekend as BA customers were forced to make alternative arrangements after their flights were cancelled.
In a statement, BA said: "Our IT systems are back up and running and our operation continues to run as planned today with a full flight schedule at Heathrow and Gatwick.
"We are extremely sorry for the frustration and inconvenience customers experienced over the bank holiday weekend and thank them for their patience and understanding.
"We are continuing to work to get delayed bags to customers as quickly as possible."
Meanwhile, the airline has come in for criticism after signposting some customers to a phone line costing up to 62p per minute.
BA's Twitter account suggested a number of passengers should call 0844 493 0797 - which can cost up to 7p per minute plus an access charge of up to 55p.
David Hickson, of the Fair Telecoms Campaign, told the Sun: "BA is being greedy and foolish. The Fair Telecoms Campaign calls on all users of 084 numbers to cease this foolishness."
The airline said it has a freephone number - 0800 727 800 - for people affected by the IT failure.
A spokeswoman added: "As a global airline that flies to and from more than 170 airports worldwide, we have call centres placed strategically around the world in the UK, Europe and Asia.
"Customers can also add phone charges into any claims they submit and we'll look at them."
Asked about the BA meltdown at an election campaign press conference in Wolverhampton, Prime Minister Theresa May said: "The IT crisis that hit British Airways is predominantly a matter for British Airways.
"I recognise the considerable disruption and problems it cost for all those individuals who had been hoping to get away for their holiday and for their break and found themselves stuck in airports and unable to travel, and I think we all feel for those people.
"It is important that, of course, British Airways has a compensation scheme for people who are travelling.
"It's up to them to sort their IT out and to ensure that they are able to provide the services that people expect them to provide as British Airways."