At least five flights approaching Heathrow were forced to land at alternative airports due to a runway closure.
United Airlines flight UA924 from Washington DC declared an emergency on Wednesday morning due to running low on fuel, according to aviation website AIRLIVE.net.
The Boeing 767 aircraft spent around half an hour in a holding pattern above Buckinghamshire before diverting to Gatwick, where it landed safely shortly after 10.30am.
Other diverted planes included four operated by British Airways. Flights from Belfast City and Hamburg landed at Gatwick, while flights from Edinburgh and Brussels landed at Stansted.
The disruption comes after hundreds of flights were cancelled over the weekend and into Monday due to a combination of Storm Dennis and a system failure.