More than 10% of UK firefighters 'not fit enough'
More than one in 10 firefighters failed to reach a recommended fitness standard in the past year, the BBC has found.
Results in Scotland were worse than the UK average, with only a third of staff in Strathclyde meeting the standard.
A Freedom of Information request found 2,890 firefighters of 24,272 failed the tests, which measure the rate of oxygen consumed during exercise.
The government says the majority of firefighters who lose fitness can improve in a few months.
The results come as the Fire Brigades Union prepares for industrial action over changes to pension rules, which will raise retirement age to 60.
The Chief Fire Officers Association is expected to produce a nationwide fitness standard next year, but currently the majority of fire services measure the "maximum rate of oxygen uptake" (VO2 max).
This assesses the maximum amount of oxygen a firefighter's body uses while exercising, in proportion to body weight.
The generally accepted standard is 42ml of oxygen per kilogram per minute. It does not test a firefighter's physical strength, which some services test separately.
Below this level, a firefighter is likely to be told to improve their fitness.
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."
Firefighters who joined the service since April 2006 already have a retirement age of 60, meaning the government's proposed pension changes would only affect around two-thirds of serving firefighters.
The proposals would extend this to longer-serving officers who are currently due to retire after March 2022, in line with police officers and the armed forces.
Dr Williams' review on firefighters' fitness found that fewer than half of those in their 50s could meet the commonly accepted fitness standard.
The Fire Brigades Union's general secretary, Matt Wrack, said the BBC's figures "reflect the fact that fitness naturally declines as firefighters age".
He added: "They demonstrate the ridiculousness of the government's plans to expect large numbers of firefighters to work until 60."
But the Department for Communities and Local Government said the same review found there was no reason why every firefighter who stays physically fit could not work until 60.
A spokesman said: "That report also found that the majority of firefighters who lose fitness can improve their fitness within a few months. Where this happens, their fire service will ensure they get the necessary support and a personal fitness training programme."
Dr Williams added: "Both sides have been selective in their quotes from the report and that is what I would expect them to do."