Was it a hijacked Jumbo Jet..? A military transport with fuel to burn..? Or the Royals' eye in the sky, watching over the Queen at Sandringham..?

The mystery aircraft was seen circling at high altitude over West Norfolk from first light today.

Different theories about what it was doing soon began appearing online.

But the Ministry of Defence said the aircraft was an AWACS warning plane on a routine training mission.

'It's not one of ours, it's a NATO aircraft doing routine training, it happens all the time,' a spokesman said.

'The joy of piloting it is that you fly a steady orbit while the team on board do their work.'

AWACS carry sophisticated surveillance equipment. Flying at 30,000ft, their radar can cover an area of 120,000 square miles, scanning for hostile aircraft.

Three of the spy planes can simultaneously cover the whole of Europe.

The MOD said the AWACS seen over Norfolk had not taken off from a base in the county and was not expected to land at one when it had completed its flight, which could last up to 10 hours.

Today's clear, cold conditions mean aircraft flying at high altitiude leave condensation trails, which can be seen from many miles away.

Similar aircraft to the NATO AWACS operate from RAF Waddington, in Lincolnshire.