London, Aug 29 : Thousands of air passengers have been left stranded after a major outage hit UK air traffic control systems.  More than 1,200 flights were reportedly cancelled due to the outage. People are stuck in the UK and abroad after National Air Traffic Services (Nats) on Monday limited the number of planes landing, the BBC reported. Airlines and airports warned there were still "significant delays" despite the issue being fixed within hours. Holidaymakers ' many of whom were returning from holidays and due to return to work on Tuesday ' were braced for the chaos to continue as airlines struggle to clear the backlog. There have been warnings that some knock-on disruption could last for days. Heathrow said late on Monday evening that schedules remained significantly disrupted, adding that people travelling on Tuesday should contact their airline before heading to the airport, the British news broadcaster reported. Gatwick Airport said it was planning to operate a normal schedule on Tuesday, but advised passengers to check the status of their flight with their airline before going to the airport. London Luton Airport also said on Monday evening that flights across UK airspace remained subject to delays and cancellation, and people should check with their airline for the status of their flight. Nats confirmed that it had identified and remedied the issue. It said that "it will take some time for flights to return to normal". Several UK airports and airlines including British Airways, Virgin Atlantic and TUI, were hit by delays and cancellations on Monday. /IANS