Flights from Heathrow, Gatwick and London City airport could be disrupted due to a snowstorm forecast for later today.
The warning was issued by British Airways, saying that the poor weather forecast could limit the number of services allowed to use Heathrow in particular.
More than 60 flights have reportedly been cancelled from the London hub.
Heathrow said: “Wintry weather is forecast at Heathrow and across the UK and airlines will be adjusting their schedule as a result.”
Airlines are offering passengers the option of rebooking flights free of charge.
BA said: “As a result of the forecast poor weather we have agreed with Heathrow, National Air Traffic Services and other airlines to reduce the afternoon and early evening flight schedules at the airport on Thursday.
“The number of aircraft permitted to land by NATS at the Heathrow from 3pm is likely to be significantly reduced as the airport’s ground teams may need to remove snow and ice from the airport’s two runways and multiple taxiways.
“Many aircraft will also have to receive de-icing treatment prior to departure to ensure that they are safe to take off. The safety of our customers and crews is our top priority at all times.”
Flights to and from Gatwick and London City airports may also be affected.
“We are sorry for the difficulties which are likely to be caused by the poor weather and will do all we can to minimise the effect it has on our operations.”
BA urged passengers not to travel to the airport unless they have a confirmed booking on a flight that is operating.
“We understand that you may wish not to travel to or from London Heathrow on Thursday even if your flight is scheduled to operate as planned,” the airline added.
“You are able to move your flight to an alternative time/date between Wednesday 11 January and Sunday 15 January (inclusive) if you are due to travel on a short haul service that is scheduled to depart from or arrive at London Heathrow between 15:00 GMT and 23:59 GMT on Thursday 12 January.
“Please keep checking the latest information about your flight on a regular basis before travelling to the airport, as the weather forecast may change. You can also check your flight status on your mobile phone.”
Virgin Atlantic said: “Due to the forecast weather conditions in south-east England, customers booked to travel from London Heathrow on 12 January 2017 who prefer to rebook may do so, without penalty.
“Rebooking is subject to availability in the same cabin class for flights and travel is to have commenced by 14 January 2017.”
Flybe said: “Poor weather has been forecast over the next few days across northern England, Ireland and Scotland, followed by snow showers affecting Germany and Italy, which could potentially impact travel.
“Flybe, like every airline, has minimum acceptable weather conditions in which we can operate safely. You can be assured that we will be doing our best to run our services uninterrupted.
“We would like to apologise for any inconvenience this possible poor weather might cause you.
“Whilst we cannot control the weather, we will always do all that we can to get you to your destination as soon as is safely possible.”
EasyJet plans to operate its Thursday flying programme as scheduled. “We are closely monitoring the weather for any changes to the forecast,” the airline said.