British Transport Minister Mark Harper said on Tuesday it would take several days to resolve widespread disruption to flights to and from the country after air traffic control systems were hit by a technical problem, Reuters and News.ro reported.

Hundreds of flights were canceled or delayed on Monday – one of the country’s busiest days – after air traffic controllers were forced to switch to a manual system.

Harper added that government officials do not believe the technical problem was the result of a cyber attack.

“There will be some impact today and I suspect it will be for a few more days as the airlines bring in their planes and get their services back to normal,” Harper told the BBC on Tuesday.

Heathrow Airport, the UK’s busiest hub, advised passengers on Tuesday to contact their airline before traveling to the airport.

Airlines said they had adjusted flight schedules to try to carry as many people as possible, but some planes and crews were not where they were supposed to be.

“We are making every effort to return affected customers,” British Airways said.

Passengers are encouraged to inquire about possible delays or delays with departure airlines or airports.