Ryanair will add more than 500 flights serving London Stansted during the October half-term, as the airline's boss branded Heathrow "hopeless" after it extended its cap on passenger numbers.
The low-cost operator said the move will provide capacity for an extra 100,000 travellers at the Essex airport.
It follows the decision by Heathrow to continue flight restrictions, introduced in July because of staff shortages, until the end of October.
The limit of 100,000 daily departing passengers had been due to end on 11 September.
The airport said without a cap, it will not be able to safely and reliably handle its flight schedule.
But the extension will hit families' plans for half-term getaways.
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 midterm break."
Ryanair insisted that it and Stansted have "more than sufficient staff to handle these additional flights".
Heathrow has 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.
But flight operators have blamed the airport - the UK's biggest and one of the busiest in the world - for failing to prepare for the bounce back in passenger numbers following the COVID pandemic.
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.
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."
Last week, Heathrow's chief executive John Holland-Kaye said that the airport was beginning to recover from the travel chaos of recent months.
Thousands of jobs were lost in the aviation industry during the pandemic, when travel restrictions saw most flights grounded, and there has been a race to recruit new workers to cope with the resurgence in demand.
Training staff and securing the necessary security clearance has also proved a lengthy process.