Figures obtained by the BBC show there are now 2,101 full time firefighters serving west, north and south Yorkshire, compared to 2,658 in 2010.
In total, the budget for the fire services has been axed by Â£21.9m over the past five years with a further Â£15.2m due to be slashed by 2020.
The Fire Brigades Union (FBU) said cuts had put crews in a "moral dilemma".
Pete Smith, FBU Yorkshire secretary, said: "We just don't have the man power anymore when we turn up on a job and ultimately, as harsh as it sounds, that will lead to deaths.
"It's putting firefighters in a moral dilemma because the instinct will always be to run into a burning building.
"It's time that fire chiefs started holding the government to account."
Reduced public funding has led to firefighter cuts and a government push for emergency services to work more closely together.
Dave Walton, deputy chief fire officer for West Yorkshire Fire and Rescue Service, said: "We've seen austerity starting to bite.
"In West Yorkshire, we've taken the approach of tackling it head on. We've accepted it as an economic and political reality."
As well as fewer full time firefighters there are fewer retained firefighters as well as control room and support staff.
Mike Penning, minister for policing, fire, criminal justice and victims, said over the past five years fire services had delivered significant savings while improving outcomes for the public.
"Since 2010, fire deaths have fallen by 21% and fire injuries requiring hospital treatment have fallen by 26%."
He said fire services could continue to deliver savings by working with other emergency services as well as reducing back office functions.
A fire union has warned a 21% reduction in the number of frontline firefighters across Yorkshire will lead to deaths.