The union said local authorities should be given powers to levy taxes on under-used holiday houses, or even buy them, pointing out that a house which is only used for a few weeks a year is very different to one which is occupied most of the time.
More than 40,000 people have a holiday home in the South West, with almost 32,000 living outside the region, said the study.
Almost 30,000 people own a holiday home in Wales. More than 10,000 people from outside Cornwall own a home in the county, while the figure for Gwynedd in North Wales was 7,700 and almost 5,000 in north Norfolk.
Paul Kenny, general secretary of the GMB, said: "In many areas urgent action is needed to ascertain if properties used as holiday homes are actually in use at all.
"A holiday home that is only used for a few weeks a year is very different to a holiday home that is occupied for most of the year in terms of its economic benefits to any locality.
"A holiday home that is used only a few weeks a year at a time when there are families in bed and breakfast accommodation gives rise to fundamental questions on the role and power of the local authority on the use of residential property in its area.
"We believe that under the Localism Act, local councils should have the power to levy taxation on under-used holiday homes and other empty properties.
"In areas with acute housing need, questions should be raised in the council chamber as to whether under-used houses should be subject to compulsory purchase."
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