More than 40 flights have been cancelled or diverted because of a lack of staff in air traffic control at Gatwick Airport.
The airport apologised for "any inconvenience caused" and urged passengers to contact their airline for information.
A spokesperson for Gatwick Airport confirmed 42 flights had been cancelled or diverted, while dozens more were heavily delayed on Thursday.
They said: "The situation is however improving with an additional air traffic controller now in place.
"The air traffic control restrictions are reducing as a consequence and more aircraft are able to arrive and depart."
More than 6,000 passengers are likely to have been affected by the disruption.
National Air Traffic Services (NATS) had earlier said "air traffic control restrictions have been put in place" due to "a short notice staff absence" affecting the air traffic control team at Gatwick.
"We are working closely with the airport to ensure we can handle flights with as little disruption as possible and we apologise very sincerely to people who have been inconvenienced [as a result of unavoidable diversions]," they said.
One person complained on social media that a flight had been diverted to Bournemouth airport, while another said they had to take off from London Stansted.
Laura Neary, 29, was due to catch a Ryanair flight to Dublin at 5.30pm but it diverted to London Stansted, which she had to travel to by coach.
Ms Neary, who was travelling on her own, said some passengers received text messages saying they would need to take a coach to reach the airport, in Essex, while others were told they could still board the flight from Gatwick.
The sales worker from the Irish capital told the PA news agency: "I don't even know if I can get back to Dublin tonight."
Ryanair boss calls for NATS chief to resign
Ryanair boss Michael O'Leary called on NATS chief executive to resign.
"Airlines are paying millions of pounds to NATS each and every year and should not have to see their passengers suffer avoidable delays due to UK ATC staff shortages," he said.
Julia Lo Bue-Said, chief executive of Advantage Travel Partnership - a network of independent travel agents, said: "The situation at Gatwick is unacceptable. This kind of disruption causes havoc for travellers and has huge financial implications for airlines, travel agents and the entire ecosystem.
"There needs to be an urgent inquiry into why there appears to be staff shortages in this crucial area, and measures implemented to stop these incidents occurring again."
EasyJet 'very disappointed'
EasyJet said: "We are very disappointed that customers are once again impacted by this - and while this is outside of our control, we are sorry for the inconvenience caused to our customers.
"We are doing all possible to minimise the impact of the disruption, notifying those on cancelled flights of options to rebook or receive a refund, and provided hotel accommodation and meals where required."
The Sussex airport said it was "working closely with NATS to build resilience in the airport's control tower to ensure disruption is kept to a minimum".
"NATS are a world-class provider of air traffic services and London Gatwick's senior management recognises how hard the airport's air traffic controllers are working to keep the operation moving," they added.
Bank holiday disruption
It comes after the NATS control system for the entire UK was hit by a technical glitch over the bank holiday weekend, causing widespread disruption.
More than a quarter of flights to and from UK airports were cancelled, affecting around 250,000 people.
Cancellations continued for two more days as planes and crew were out of position.
Rory Boland, of consumer group Which?, said: "It is unacceptable that some Gatwick passengers have been hit by further air traffic control problems so soon after the chaos a few weeks ago.
"This is not an issue caused by airlines, but they must meet their legal obligations to look after passengers and provide them with support during delays and help with refunds and re-routing - including with other carriers if necessary.
"To help end this cycle of miserable passenger experiences, the prime minister must play his part and prioritise legislation to give the Civil Aviation Authority stronger enforcement powers."