Long queues have been reported at airports, and British Airways says some of its flights are “likely to experience disruption”.
The UK’s largest airport, Heathrow, said the outage was affecting “select systems”.
“Flights are operational and we are implementing contingency plans to minimise any impact on journeys.”
Some flights into the UK have also been disrupted. Jemma Wheeler, 30, said her family of five was stuck at Palma de Mallorca Airport after the IT issues caused delays at check-in.
“We thought it was a quick, two-hour flight and we would be back in no time to enjoy the nice weather at home,” she told the BBC.
Those waiting for their flights are being urged to keep an eye on communications from their airlines and to keep essential items in their carry-on if they check in bags.
Train services are still running, although the National Rail Enquiries website has warned of “widespread IT issues across the entire network”.
It said this could potentially lead to “short-notice train cancellations”.
“Additionally, other key systems, including real-time customer information platforms, are also affected,” it added.
It said services run by Gatwick Express, Great Northern, Southern and Thameslink may see alterations and cancellations.
British TV channels Sky News and CBBC are now both back on air after having been affected.
