The latest passenger numbers at Prestwick Airport show a fall of 40%.
The number of people using the airport dropped from 101,032 in January 2010 to 60,993 in January 2011.
John Scott, Scottish Conservative MSP for Ayr, said:
"These figures are extremely concerning.
"What must be appreciated by authorities and governments across the UK is the strategic importance of Prestwick Airport, which is nearly always free of fog and ice and available for the use of less fortunate airports across Scotland.
"Prestwick over the past two winters has kept Scotland's airspace open for business and its vital strategic significance must on that basis be recognised."
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