LONDON — Flights departing from the U.K. have been have been grounded following a “technical issue,” London’s Heathrow Airport said in a statement Tuesday. It added that it was working with Britain’s National Air Traffic Service (NATS) to resolve the issue. “We are investigating a technical issue which is causing some disruption to flight departures,” NATS said in a separate statement, adding that engineers were on site and would provide and update when they could. “Arriving flights are not currently impacted,” Heathrow said, although it advised passengers to check flight times. With 84.5 million passengers last year, Heathrow ranked seventh in the table of the world’s busiest airports, according to data published by Airports Council International.