A spokesperson said: “Some potential delays to/from London Stansted and Luton airports today due to fog.
“Affected passengers will be notified and any passengers travelling should check their Ryanair app for flight updates before travelling to the airport.
“We regret any inconvenience caused to passengers by these thunderstorms which are outside of Ryanair’s control and affect all airlines operating to/from London today, Saturday September 16.”
Passengers travelling on flights to and from London Stansted and Luton airports could be impacted by delays.
Adverse weather has impacted flights in recent weeks with storms, fires and fog causing delays.
Ryanair issued a warning for passengers travelling to and from Spain earlier this week, due to thunderstorms.
TUI issued a general warning for passengers travelling to top destinations due to the impact of adverse weather across Europe.
A spokesperson from a holiday company said: “We are currently seeing unusual adverse weather conditions in a small number of destinations across Europe which we know is causing concern for some of our customers due to travel.
“Please be assured that we actively monitor weather conditions in all the destinations we visit to be sure it is safe for our customers and colleagues.”
It said that customers would be contacted with advice for affected customers if there was an issue.