Air passengers have been warned to expect further disruption as volcanic ash is again blown into UK airspace.
The Department of Transport warned that some flights could be grounded from Sunday, with restrictions possible until Tuesday.
The announcement was bad news for passengers hoping the skies were getting back to normal a month after ash from the erupting Icelandic volcano Eyjafjallajokull first halted UK flights.
Different parts of British airspace may be closed within the next few days and travellers have been warned to check with their airline before setting off for the airport.
Nats said a "high density volcanic ash cloud" is rapidly approaching Northern Ireland, resulting in the closure of air space over parts of the province.
But Belfast International and Dublin airports will remain open until 7am, it said.
Nats added: "There are currently no other restrictions within UK airspace. We are working closely with the Met Office and with the CAA, which agrees no-fly zones based on Met Office data."
Some of Britain's busiest airports in the South East, such as Heathrow and Gatwick, are expected to be affected. The wind direction is expected to change in the middle of next week and will blow from the south west, taking ash away from the UK.
The warning came as the Met Office began to publish five-day forecasts for the first time which predict the movement of the ash clouds.
Transport Secretary Philip Hammond said: "The Government is carefully monitoring this situation and the safety of passengers will remain our paramount concern. We have taken the decision to publish five-day forecasts as we want airlines, other transport providers and the public to have the best possible information. However, the situation remains fluid and these forecasts are always liable to change.
