The alarm was raised shortly after 1.30 am when the man, not yet identified by police, arrived at the former nuclear command centre in a digger apparently stolen from Cupar earlier this week. Apart from police, one of the first people at the scene was the bunker’s manager Jim Braid, who said that he had received a call at around 1.45 am when the alarms went off.

He said that although there had been a digger used to travel to the bunker, no damage had actually been caused. The man had managed to break through steel shutters and enter through a window.

Mr Braid said it appeared the man had worked his way in to the bunker itself, and that he had been seen on CCTV monitors. It appeared he had visited various parts of the building, and had even dressed in some of the uniforms and had taken weapons from the walls.

Mr Braid said that he had been able to show police the ways in and out of the building and he had then been kept away from the scene.

He added, however, that there was enough food in the building to last the man for weeks and there were beds, toilets and water.

North East Fife’s most senior police officer, Chief Superintendent Alan Maich held a Press conference at Anstruther Police Station in mid-morning.

He said that the man involved had “holed himself up” in the premises. He said that because of the nature of the premises and the fact there are deactivated, replica and bladed weapons, police were exercising caution.

The superintendent said that it was not certain at the moment that there is no live ammunition on the premises and that the firearms team had set up a cordon.

The officer said that because the premises were built partly to keep people out, the situation was made more complex and he confirmed that the man had access to food and accommodation. He said police did not know who the man was although lines of inquiry were being followed. He said he could not confirm, due to operational reasons, whether or not the man had been spoken to.

Police hope that, with the firearms squad and trained negotiating teams in attendance, the matter can be brought to a close without anyone being injured.

The bunker, now a popular tourist attraction, was built in secret in the 1950s and would have housed senior ministers and military commanders in the event of an imminent threat to the country.

It is topped by a traditionally-built farmhouse and has 24,000 square feet of accommodation on two underground levels.

The bunker, designed to keep up to 300 people alive for several months, is encased in three metres of concrete.

Early this afternoon the situation at the Secret Bunker was unchanged with the man still deep inside the complex.