Britain's air traffic authority NATS said that it has fixed the issue. (Representational)

Britain's air traffic authority NATS said it had done enough to fix an earlier system issue for flights to start returning to normal after technical problems delayed planes at Britain's two busiest airports, London's Heathrow and Gatwick.

The air traffic control issue had compounded travel chaos in London, with commuter trains impacted by a summer heatwave and the Eurostar rail link to France halted by a power cable failure.

"Following the technical problem at Swanwick air traffic control centre this morning, we have now fixed the issue sufficiently to safely increase traffic flow rates and will see an improving picture through the rest of the day," NATS said in a statement.

Earlier, Eurocontrol, NATS' pan-European equivalent, said the outage had caused a high level of delays at Heathrow and Gatwick, though they appeared to be easing.

British Airways said it had been affected by severe weather conditions overnight as well as by the technical issue.

"We are doing everything we can to minimise the disruption, and our teams are working tirelessly to ensure as many of our customers as possible are able to depart on their trips," it said in a statement.

"We have apologised to our customers, and are rebooking them onto alternative services or offering them hotel accommodation if necessary."