Airline passengers may face travel disruption over the next two days due to expected strikes across European Air Traffic Control services.
easyJet said it expects the strikes in Italy and Portugal - the timings of which are expected to differ from country to country - will have "a significant impact on our flights during these periods".
Both British Airways (BA) and easyJet said they have re-timed a number of flights, with BA also using larger planes to help customers affected by cancellations.
BA has cancelled a number of flights to and from Italy, a spokeswoman said.
The airlines advise customers to check their websites for the status of their flights, with easyjet adding that delays are possible due to knock-on impact of the strike.
In a statement easyJet said the Italian Air Traffic Control personnel are on strike from 1pm local time until 5pm local time, while the Portuguese controllers are on strike between 7am to 9am local time and 2pm to 4pm local time.
The airline added: "Like all airlines flying to/ from/and over these countries, we expect it will have a significant impact on our flights during these periods.
"easyJet would like to reassure passengers that it will do everything possible to minimise any inconvenience for passengers.
"It has pro-actively re-timed some flights later into and out of Lisbon in order to avoid the strike period as well as re-timing some flights to and from Milan Malpensa.
"We will advise all affected passengers on these flights via e-mail. Customers can also check these re-timed flights on our Flight Tracker page on our website."
A spokesman for BA said: "We are doing all we can to minimise disruption to customers affected by air traffic control strikes across Europe.
"We have re-timed a number of flights and are using larger aircraft where possible to help more customers, from cancelled flights, fly to where they need to be.
"We are advising customers on short-haul services to keep checking the very latest information on our website as the nature of the industrial action may change throughout the day."
easyJet said the industrial action is likely to continue into Thursday with the French Air Traffic control staff taking part.
Air traffic controllers in the UK will be working as normal tomorrow and on Thursday, despite industrial action being planned across Europe, NATS said.
NATS chief executive, Richard Deakin, said: "NATS controllers will be working as usual in spite of the threat of industrial action in Europe.
"If the strikes go ahead we will work closely, as ever, with Eurocontrol and other European air navigation services to help keep people moving whenever possible.
"We will also work closely with our airline and airport customers to help them manage their operations should any changes be necessary.
"Any passengers who are worried they might be affected should check with their airlines for the latest information."
The industrial action is being staged by two organisations in protest against the European Commission's Single European Sky initiative, which aims to consolidate air traffic services across the continent, NATS said.
The Air Traffic Controllers European Unions Coordination (ATCEUC) - an umbrella group that represents 14,000 members across 28 countries - is calling the strike for tomorrow, before members of the European Transport Workers' Federation (ETF) then walk out on Thursday.
ATCEUC previously called for a 'Day of Action' in October last year before cancelling it following assurances of further negotiation.
The European Commission says the Single European Sky programme will triple the region's airspace capacity, cut costs and reduce delays.