The fraught question of whether the size of a fireman's girth is the best guide to his fitness for the job could result in a walkout by his colleagues, after a union decision to ballot for strike action.

Kevin Ogilvie, 46, who has been a firefighter in Aberdeen for more than 20 years, was dismissed after his weight rose over 20st (127kg) and he failed an annual fitness test. His employers informed him this week that he posed a safety risk and added that all crew had to be able to perform “the requirements of a modern firefighter”.

Colleagues, however, have sprung to his defence. According to the Fire Brigades Union, which has requested a hearing on the case, Grampian Fire and Rescue in Aberdeen dismissed him without an appeal process. If the dispute is not resolved, firemen in the city could vote for strike action by the end of this month.

Roddy Robertson of the FBU said that Mr Ogilvie, who only this week was awarded a 20-year long service medal, was told to lose 3st in three months. “I don't think any medical person would sanction that kind of weight loss in such a short period. Clearly he is overweight, there is no argument about that. But the issue is not about his weight but about his fitness,” he said.

Mr Ogilvie, whose precise weight has not been disclosed, says on his Bebo social networking web page that he works for Grampian fire brigade and enjoys going out for “a beer or two” with his mates.

Grampian Fire and Rescue said that it had been forced to act after “a very long and arduous process of support and consideration”.

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