Air traffic control company Nats denies it faced Government pressure to ease flight restrictions following the spread of the volcanic ash cloud.
The Civil Aviation Authority stepped in to announce that airports could reopen after safety tests showed engines had "increased tolerance levels in low ash density areas".
A CAA spokesman said Nats was acting on guidelines that prohibited flights through any type of volcanic ash cloud when it imposed the restrictions last week.
But in light of the new research, the CAA effectively moved to revise the advice last night. This meant Nats had the go-ahead to resume operating as an air traffic controller, with the first flight landing at Heathrow just before 10pm on Tuesday night.
The CAA, the regulator which oversees Nats, said it had worked with the company throughout the restrictions. But the CAA spokesman said it always had overall control of the matter.
Nats spokesman Alex Bristol said the only pressure the company had been under was to understand the nature of the volcanic ash cloud. He said that information about how aero engines could cope with adverse ash conditions had only just become available.
He added: "Our first priority has been safety and the reason we didn't simply lift the flying restrictions was because of our desire for safety."
Asked if Nats had been relying on old guidelines in judging safety conditions, Mr Bristol replied: "We had a number of sets of guidelines. It's taken the whole of Europe this long to grapple with all this. It's taken a while to produce evidence necessary to establish safety levels."
He also denied Nats had ignored interested parties, saying: "We have listened to all of the evidence coming in. There's been a lot of different evidence. One test flight led to damage to a military plane."
