LONDON (AP) — Dense fog in the greater London region has caused delays to holiday travelers at several U.K. airports.

Officials said Thursday there were significant fog-related delays at Heathrow, Gatwick and London City airports. British Airways advised travelers to check their flight status online before heading to the airport.

The company said air traffic controllers had to allow greater space between planes because of the low visibility, leading to a reduced number of takeoffs and landings per hour.

The fog cleared up by late morning but officials warned that some delays would continue throughout the day.