Lifeline Services
5 live Investigates submitted Freedom of Information requests to 52 fire services asking how many firefighters had failed tests between May 2012 and April 2013.
Of these, 36 fire services provided figures, which have not previously been compiled and published. Of 24,272 tests, some 2,890 firefighters failed.
Some 665 firefighters failed to reach a lower standard (35ml of oxygen/kg/min), below which a government-sponsored review published in December 2012 said someone was at risk "of sudden death particularly while undergoing high levels of physical exertion".
Dr Tony Williams, who wrote the review, said at this standard "some firefighters may simply not be fit enough to meet the physical demands of the most arduous tasks they are likely to have to undertake".
Some fire services said their firefighters may have taken the test more than once.
'Arduous physical work'
In Strathclyde, of 652 tested, there were 435 "unfit" firefighters, of whom 111 were "seriously unfit".
The Scottish Fire and Rescue Service (SFRS) said it could not explain why Scotland's figures were worse than the rest of the UK but it has started a review of working practices.
A report published in 2010, based on the most recent Scottish Health Survey showed that a higher proportion of men and women were overweight, including obese, in Scotland than the rest of the UK.
Professor Kevin Sykes, who advises UK fire services on fitness, said it was well established that being in good physical condition minimised the risk to health.
He added: "Firefighters are asked to carry additional weight, which can have a dramatic effect on their ability to perform arduous physical work, particularly for the less fit."