Two sea forts designed to defend the south coast have been out up for sale by Bucks businessman Mike Clare - and are expected to fetch more than £8 million.
The island properties, surrounded by water, were commissioned by Lord Palmerston in the 1860s and took 20 years to build.
However the Solent Forts, located near Portsmouth, were never used in battle and now feature luxury hotels.
Buckinghamshire-entrepreneur and founder of the Dreams bed company, Mike Clare acquired the properties in 2009 and 2012 and spent three years painstakingly redeveloping each fort to create two of the world's most unique hotels.
After stepping back from the business approximately three years ago, Mr Clare has decided to sell these properties in order to focus on other businesses and charity interests.
On behalf of the seller, Colliers is inviting offers for the freehold interest in these landmark properties and expects to soon announce a bid deadline.
The properties are estimated to reach a combined sale price in excess of £8 million.
"In keeping with the unique nature of the forts and aside from buyers looking to continue the current use, we are expecting and would welcome interest from buyers looking to re-purpose the properties to say, ultra-private residences, corporate retreats or exclusive use venues," said Paul Barrasford, director in Colliers International's Hotels Agency team.
"This is an incredible opportunity to acquire two of the most remarkable properties to come to the market in recent years.
"Commissioned by prime minister, Lord Palmerston, during his tenure between 1859 to 1865, the forts were built as a line of defence against enemy attacks on the Solent and Portsmouth.
"They took 20 years to build but were never used in battle as intended - instead they were re-purposed as temporary army barracks before being decommissioned by the Ministry of Defence in 1956."
Known as the Solent Forts, the properties, No Man's Fort and Spitbank Fort, are available as combined properties and businesses or on an individual basis.
No Man's Fort has been transformed to create a four-star island hotel venue.
With a maximum capacity of 200, guests can hire the entire venue exclusively for weddings, corporate events and celebrations.
The property has 23 en suite guest bedrooms/suites, staff quarters, a restaurant, five themed bars, laser battle play area and a spa.
It also features a lighthouse with private dining, rooftop hot tubs, sauna, fire pit, hot pool and helipad.
Spitbank Fort is an even more upmarket offering with nine luxurious guest suites plus staff accommodation, events spaces including a restaurant, two bars and a "wine cave".
Totalling some 33,000 sq ft over three floors with 15 foot thick granite walls, the fort has a maximum capacity of 60 and features an open central courtyard, plus roof top terrace including sauna, fire pit and hot pool.
The portfolio also includes a third fort, known as Horse Sands Fort, which is being sold by auction with bids starting at £750,000 through Clive Emson Land and Property Auctioneers.
Horse Sands Fort is the same size as No Man's and shares the same compelling past.
The property is currently being used to showcase the incredible history of these forts and the owner has preserved the beauty of this historic monument with its 100 chambers and living quarters, original gun carriages and armour plated walls.
It previously had planning permission for 22 luxury residential apartments.