There are fears that boat owners living on Thorpe Island will be made homeless as eviction plans get underway.
Letters sent to people moored at Jenner's Basin, in Thorpe St Andrew, state they have until the end of April to move out.
Should they fail to do so, the notice warns that all property will be 'disposed of' after May 1.
Meanwhile, security personnel has been drafted onto the island, near Norwich, to prevent any future access.
It comes as the seven-and-a-half acre site was sold on Friday for just £200,000 by its former owner Roger Wood.
The island has been at the centre of a decade-long legal wrangle between the 74-year-old and the Broads Authority.
But that has now come to an end and the basin, which was valued at £2m last summer, is now under new private ownership.
The move has been condemned by the Save the Island campaign group, which was set up last year to protect boat owners.
A spokesman said: 'We believe this is about as good as ethnic cleansing. It is a way of life which is being taken away.
'Some of the guys down at Jenner's Basin work in the city, and they are now faced with not only losing their homes, but also losing their jobs.
'The primary concern is that there are no available moorings at River Green, so there is nowhere for the boat owners to go.'
There are around 14 vessels currently moored at the basin, two of which are occupied permanently.
A notice sent to each boat owner states they will be permitted to enter the site, but will be recorded upon entry and exit.
Should they refuse to provide their details, the letter warns they will be denied access.
One owner, whose boat is still moored on the island, said he had been told to move it by security patrolling the area.
The 54-year-old, who wished to remain anonymous, said: 'I told them my boat was there and he [the security guard] said move it.
'When I asked him where I should move it to, he said 'I don't care'.'
He said one of the biggest issues was the cost of moorings elsewhere in the Norwich area.
'I just feel sorry for the guys who are down there on the live-a-boards,' he added.
The new owners have been contacted for comment, but are yet to respond.
Why it was sold
Former tugboat captain Roger Wood sold the island after a decade-long legal battle with the Broads Authority left him owing thousands of pounds.
The 74-year-old has always argued that he could legally moor boats in the basin.
However, the case has been before two planning inspectors and a high court judge, who have all deemed the development unlawful.
He purchased the island for £460,000 in 2007 with an aim to make money from berthing fees in his retirement. But years of court battles have forced him to remortgage his home.
A spokesperson for the authority said Mr Wood failed to obtain planning permission for the developments he undertook.
The letter
The letter sent to boat owners by Great Yarmouth-based Kodiak Security states the site is to be managed 'full-time' by personnel.
It said: 'Only the registered owners of the currently moored vessels will be permitted to enter the site, except by prior agreement.'
The notice states any verbal or physical abuse to security staff will result in being 'permanently removed' from the site.
It adds there is no planning permission to moor in or around the site and that a high court injunction 'was now in effect' to clear the area.
Boat owners have been told they have until April 30 to remove their vessels and possessions.
'If your property is not removed by this time, it will be disposed of after 1st May 2017,' the letter states. Please note that all costs related to the disposal will be sought from you as the registered boat owner.'
Vehicles will not be allowed onto the site after April 10.
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