More than 100 British Airways flights at Heathrow were reportedly cancelled due to an IT glitch ahead of the busy late May bank holiday weekend.
Aviation data firm Cirium calculated that 5% of all services were affected by 5pm on Thursday, equating to 43 flight cancellations.
Numbers continued to rise in the evening to reach three figures, with more than a dozen European departures from Terminal 5 cancelled this morning (Friday) due to the knock-on effects.
Many arriving passengers faced being held on aircraft yesterday before being allowed into the airport terminal.
BA was scheduled to operate around 800 Heathrow flights during the day on Thursday.
The airline blamed a technical issue with flights mainly cancelled to destinations with multiple departures in an attempt to allow passengers to travel.
BA’s booking and check-in systems were also affected.
Heathrow issued an alert on its website saying: “British Airways has suffered a technical issue which is impacting some departing and arriving flights.”
The disruption came as security guards at Heathrow started a three-day strike over pay.
There are roughly 1,400 striking staff based at Terminal 5 and in campus security represented by the Unite union.
The airport has insisted that operations will not be affected.
A BA spokesperson said: “While the vast majority of our flights continue to operate today, we have cancelled some of our short-haul flights from Heathrow due to the knock-on effect of a technical issue that we experienced yesterday.
“We’ve apologised to customers whose flights have been affected and offered them the option to rebook to an alternative flight with us or another carrier or request a refund.”
The cancellations are on multiple frequency routes where possible so passengers can be rebooked onto alternative services.
Which? Travel editor Rory Boland said: “As we approach the busiest travel period since before the pandemic, it’s not a good look for British Airways to once again be cancelling flights because of an IT meltdown, especially after the significant disruption it caused travellers last year.
“If your flight with BA is cancelled this half term due to these ongoing technical issues, you will be entitled to a refund or rebooking at the earliest available opportunity, including with other carriers if necessary.
“Passengers are often given the runaround on this right, but it is the law, so it’s worth being insistent. It appears the technical issue in this instance is BA’s own doing, so any flight cancellations and delays of more than three hours should also be eligible for compensation.
“The Civil Aviation Authority must hold airlines to account so that passengers are treated fairly when things go wrong. Carriers need to face consequences when they flout the rules, and the CAA should be given powers to levy fines.”