Thousands of travellers have been stranded after British Airways' online check-in system crashed due to an IT glitch.
The airline apologised to customers for the disruption and said its technical team was working to resolve the problem.
"We are experiencing some systems problems this morning which are affecting check-in and flight departures," it said.
"This is affecting other systems as well."
Just over a year ago, BA was forced to cancel flights at Heathrow, due to problems with a supplier's IT system.

At least 73 injured after Houthi rebels attack Saudi civilian and energy facilities
Nepal rescue focused on workers trapped in three tunnels
Indonesia reopens Jakarta airport, four others after Anak Krakatau ash clears
Russia hits Kyiv after brief pause for US envoys' visit, killing two
Kenya offers temporary amnesty to undocumented East Africans amid crackdown fears
7 Indonesian airports remain shut due to volcanic ash from Anak Krakatau
Bolivia confirms 9 deaths after military base explosion
Israeli strikes on southern Lebanese town kill 11, state news agency saysÂ
