Ryanair said it will add more than 500 flights serving London Stansted during the October half-term school holiday after Heathrow extended its cap on passenger numbers.
The Dublin-based airline said the extra flights at the Essex airport will have capacity for 100,000 passengers.
Heathrow announced on Monday that the limit of 100,000 daily departing passengers it introduced in July due to staff shortages will continue until October 29.
Aviation data company Cirium said the extension will lead to a reduction in capacity of more than one million seats, from 5.9 million to 4.8 million.
Many families hoping to get away during half-term will have their plans disrupted.
The cap was initially due to run until September 11.
Ryanair chief executive Michael O’Leary said: “While hopeless Heathrow continues to cut flights and raise fares for families, Ryanair and London Stansted continue to add flights, and offer thousands of low-fare seats for the autumn mid-term break.
“With over 500 additional flights, more than 100,000 additional seats and prices starting from just 29.99 euros (£25.29), Ryanair looks forward to welcoming thousands of additional families during the autumn mid-term break on its low-fare flights to/from London Stansted Airport.
“While hopeless Heathrow continues to mismanage air travel, Ryanair and London Stansted will continue to grow and deliver for London families, the way we have through all of summer 2022.”
Ryanair insisted that it and Stansted have “more than sufficient staff to handle these additional flights”.
Heathrow has repeatedly criticised airlines for a shortage of ground handlers at the airport, which has resulted in thousands of items of baggage not being put on to their owners’ flights this summer.
In response to the extension of the cap, a Virgin Atlantic spokeswoman said: “We are disappointed that Heathrow Airport has already decided to extend the passenger capacity cap until the end of October, as additional resources come on line every week and the airport experience improves.
“Airline customers have a right to expect their bookings will be honoured and we’re doing everything in our power to minimise disruption, getting our customers to where they need to be smoothly.
“We urge Heathrow to provide a comprehensive plan for returning to normal operations as soon as possible.”
Heathrow was unable to provide figures on the number of people who have already booked half-term flights to and from the airport.
British Airways and Virgin Atlantic are reviewing their schedules for the period but no decisions on further cancellations have been made.
Paul Charles, chief executive at travel consultancy The PC Agency, said: “The Heathrow plan may smooth out journeys on the day of travel but it will cause frustration and stress for those 1.1 million passengers whose flights will be cancelled during the run-up to and over half term.
“The ongoing uncertainty, created by Heathrow, over whether your flight may be cancelled is putting off people and encouraging them to either switch to other airports or not to travel overseas at all.
“Staffing is improving at our airports and among ground handlers but it looks like Heathrow is prepared to follow this approach for many months yet in order to give passengers a better experience.”
Heathrow chief commercial officer Ross Baker said: “Our primary concern is ensuring we give our passengers a reliable service when they travel.
“That’s why we introduced temporary capacity limits in July which have already improved journeys during the summer getaway.
“We want to remove the cap as soon as possible, but we can only do so when we are confident that everyone operating at the airport has the resources to deliver the service our passengers deserve.”