The union is calling for new tax rules to tackle what it calls ‘purposefully unoccupied dwellings’ which could be used to solve the housing crisis.
It said there are 82,477 vacant properties in the South East, over a quarter of which have been vacant for more than six months.
According to GMB’s figures, in Brighton and Hove there were 3,780 vacant dwellings in 2015, the authority in the South East with the highest number.
Next highest is Medway with 2.910 vacant dwellings, Southampton with 2,633 and Portsmouth with 2,586.
The figures were taken from a 2015 study of all 67 local authority areas in the South East, compiled by GMB Southern from government data.
But Brighton and Hove City Council said its most recent figures showed only 702 properties in the city had been vacant for more than six months. It did not reveal the figures for vacant properties under six months.
Councillor Anne Meadows, the chair of Brighton and Hove City Council’s housing and new homes committee, said: “Our most recent figure for the amount of properties empty for more than six months in Brighton and Hove is 702, or around 0.5 per cent of housing in the city.
“At this key measure we are among the very best performing authorities in the south east.
“Our empty homes team does an exceptional job bringing empty properties back into use. Above all we offer empty property owners advice and encouragement, but we also use enforcement where necessary.
“We fully utilise tax disincentives, charging people 150 per cent council tax when a property has been empty for more than two years – the maximum possible at present.
“Over recent years our approach has enabled us to return more than 150 properties back into use every year – a figure that is the envy of other similar-sized authorities both locally and nationally.”
After releasing the regional figures, the GMB union called for measures to tackle the number of vacant properties in the South East.
Paul Maloney, GMB Southern regional secretary, said: “It is abundantly clear that the current tax regime is not working and a more robust structure is needed. Even if the full 50 per cent extra charge in council tax is levied, this is not a deterrent to the wealthiest investors.
“Many, many more of these 82,477 purposefully unoccupied dwellings in London must be utilised more thoroughly. These empty properties can be used and transformed into homes for people and families desperately in need of decent and affordable housing. For that to happen there needs to be a punitive tax regime put in place.
“The decisions of the Thatcher government in the 1980’s to sell council housing stock, and not replace it, and to pay landlords housing benefit instead of providing social housing directly has been a huge and expensive mistake.
“Last year, for example, £24 billion was spent on housing benefit, with much of this public money ending up untaxed in bank accounts in offshore tax havens. If a fraction of that amount had been spent on social housing for rent, the strain on the tax payer would be less and people would have housing they can afford to live in.
“The situation will only get worse next year when the Housing and Planning Act comes into force in April 2017 which will force councils to sell off high value council properties when they become empty. Selling social housing, at a time of dire shortages of homes for rent at affordable prices, is scandalous and irresponsible madness.”