In 1942 the British government built several Fort Roughs off the coast of Essex in order to guard the country from a German invasion. Fort Roughs comprise a floating pontoon base with a superstructure of two hollow towers joined by a deck upon which other structures could be added, such as a gun deck, an upper deck and a central tower unit which housed the radar equipment. The two twin towers are divided into seven floors that provide dining and sleeping accommodation and storage areas. The base was then intentionally flooded so that it sank to the bottom of the sea and the deck is visible above the sea, so in essence it is a sunken ship.

One of these forts, which eventually became Sealand, was located in international waters and its rulers therefore claim it was built illegally. On 2 September 1967 former English major Paddy Roy Bates formally claimed the man-made fort and settled there with his family.

After negotiations with English lawyers, Bates proclaimed the island his own state and himself as its absolute sovereign ruler, declaring himself and his wife Prince Roy and Princess Joan of the Principality of Sealand.

In 1968 the British navy attempted to reclaim Sealand, but were turned away when Bates fired warning shots at them.

In 1978 Professor Alexander G Achenbach, Bates' appointed prime minister, along with some Dutch businessmen, attempted a coup.

Bates, employing the services of some of his own men, later reclaimed Sealand and imprisoned the captives as prisoners of war. This led to protests by the German and Dutch governments for their citizens to be released and Bates eventually relented releasing them under the requirements of the Geneva Convention.

While Bates and his family retain British passports, Sealand has its own constitution, its own stamps, coins and currency, the Sealand dollar.

In June 2006 a fire broke out on the fort and it was badly damaged. Bates and his wife had since moved to Spain and son Michael was ruling Sealand as prince regent, sovereign pro tempore.

Early this month Prince Michael put Sealand up for sale.

"Sealand has 40 years of history dealing with different countries, " he says of its global status as a micronation. However courts in the UK, Germany and the US have questioned its legitimacy as a sovereign state and it is not recognised by any other state.

Sealand can accommodate up to 10 people but at present only security staff and caretakers are living there. The sovereignty of Sealand cannot be sold, therefore any potential buyers or investors will be obtaining leasehold or buying a tenancy and the Bates family will remain sovereign royal rulers.

Spanish estate agents InmoNaranja, who are handling the sale, have valued Sealand at Euro750m. While Prince Michael would not speculate on the amount that may be raised it is obvious that any potential bidders will be entering into a very complicated legal test case. However if its future is as colourful as its past it will be interesting times ahead.

For further information visit www. sealandgov. org or contact estate agents InmoNaranja at www. inmobiliarianaranja. es