For sale: Want to own a piece of military history? But you'll need a boat to get to it...

If you're in the market for a 19th-century military fort with a connection to D-Day and its very own moat – aren't we all? – then you're in luck.

With a guide price of £50,000, Savills are auctioning military installation Darnet Fort and the successful bidder will also become the owner of three barges, visible only at low tide, believed to have played a role in the D-Day landings.

On 10 December – just in time for Christmas – the naval fort on the River Medway in Kent will come to the market for the first time in 40 years, offering people the rare opportunity to own a piece of British military history.

But you'll need to own a boat or a kayak to visit it. 

• The 'Doomsday wreck' packed with 1,400 tonnes of explosives that could create a tsunami in the Thames Estuary
• Guardsman brings his D-Day veteran grandfather's bagpipes back to the beaches
• How a WW1 German U-boat became a tourist attraction on a British beachfront

The fort is an impressive-looking circular structure with two levels – an open upper platform and a lower level featuring a series of gun points.

However, while offered in fair condition, the magazine level is partially submerged and is accessible only by boat or kayak.

Darnet Fort is a historic scheduled monument that is protected under the Ancient Monuments and Archaeological Areas Act 1979.

The Royal Commission on the Defence of the United Kingdom constructed the fort between 1870 and 1872 as part of an inner defensive line protecting access to the naval dockyard at Chatham which is now a maritime heritage site and museum in Kent.

Although decommissioned before the First World War, Darnet Fort was repurposed during the Second World War as an observation post, with additional platforms and pillboxes added.

Originally, the fort was designed to accommodate a garrison of 100 men. 

Fort Darnet is one of only two sea forts constructed on low-lying islands within the Medway channel, alongside Hoo Island, located one kilometre upstream.

The pair are thought to have been designed to create a bottleneck defence for the river.

Originally part of Britain's extensive coastal defence system, the island was used for picnicking and leisure until the 1980s. 

Ben Fletcher of Savills Auctions said: "The sale of Darnet Fort presents a unique and rare opportunity to acquire a piece of British military history and we are excited to be bringing this landmark to our final auction of the year.

"Used historically as a place for recreational activities, the forts could serve as the base for alternative uses, subject to the necessary planning consents.

"We anticipate strong levels of interest when it goes under the hammer in December from those keen to preserve its historic integrity or simply drawn to the uniqueness of owning such a rare and individual structure."