A "technical issue" with the UK's air traffic control systems has caused disruptions and flight cancellations, leaving thousands of passengers stranded. The issue has been identified and resolved, but the backlog of affected flights is expected to continue into the week. The National Air Traffic Services (NATS) apologized for the disruption and stated that engineers are monitoring the system's performance as it returns to normal. Airlines and airports have warned of continued disruption as they work to cover the backlog of flights.