The Government said it was trying to reduce the housing stock kept for military families after it emerged that more than £25 million a year was spent on rent for unoccupied properties.
A spokesman for the Ministry of Defence said around 7,500 homes were kept vacant for service personnel and their families, at an average annual rent of £3,500 a home.
The MoD leases around 40,000 properties from a company called Annington Homes Ltd, which bought them from the military under the last Conservative government.
The spokesman said properties had to be kept available for troops being moved from base to base around the country, but added that the Government was "working to reduce the level of unoccupied properties".
"As part of our military housing policy, we are committed to have houses readily available for families when people need them, so people do not have to wait for family houses," he said.
Liberal Democrat defence spokesman Nick Harvey told the BBC he had discovered that as much as a fifth of MoD family properties were unoccupied, and described the situation as "ridiculous".
"One accepts some will be empty while they're organising people to come and go, but it's an incredible percentage that a private sector operator could never sustain," he said.
