Safe Platform to Dock Your Data
They are said to be installing computer systems on the platform, 9km off the coast of eastern England.
Their aim is to allow customers to sidestep laws being introduced in Britain and the US that allow governments to interfere with communications over the internet.
There has been no official confirmation of these activities.
The platform. called Sealand, was built so gunners could shoot at German bombers as they crossed the North Sea.
It has been occupied since 1967 by retired British army major Roy Bates and his family. He declared it a sovereign state in 1975, with its own constitution, currency and passports.
The Foreign Office refuses to recognise Sealand.
Bates's son, Michael, says he was contacted by HavenCo. based on the Caribbean island of Anguilla, with the idea of setting up the data haven "It's a marvellous concept and great fun," Michael Bates says.
"It is about freedom and liberty and making it easier to do business in private and express themselves freely."
The plan is to rent space on computers based in the platform to companies that want to run their electronic businesses free from government interference.
"We felt companies needed a defence against the prying that can be done by governments," HavenCo's 31-year old chief executive. Sean Hastings, told the Independant Daily
"I was reading a book called How To Start Your Own Country, and Sealand was in it. I thought it sounded perfect, so I sent off an email."
He said his company would not accept business from child pornographers or hackers who wished to launch attacks on other computers.
Sealand, a concrete platform perched on two towers, is accessible only by helicopter or boat. Bates moved in after it was abandoned in the post-war years. When the Royal Navy sent ships to investigate, he fired warning shots at them. He was prosecuted, but successfully defended himself on the grounds that Sealand was outside British territory.
And Sealand is still prepared to repel intruders. Armed guards patrol the landing deck. and there is enough food and fuel stored to keep the computers and personnel going for a year.
However. last month the principality was implicated in an arms trafficking case. Spanish police said a gang claiming to represent Sealand tried to buy Russian arms and export them to Africa.