The Combined Fire Authority voted to cut the crew size - to save around £700,000 per year - despite opposition from the Fire Brigades Union (FBU).
It’s understood 17 full-time firefighter posts plus a dozen on-call positions will be lost.
As part of the cut-backs the fire service will also remove a second engine from Oakham. The remaining Oakham engine will have a crew of five.
The authority is exploring the possibility of using £150,000 from Rutland County Council to introduce a rapid response vehicle which may be based in the Rutland area.
But Leicestershire FBU spokesman Graham Vaux claimed lives would be put at risk as a result of the authority’s decision.
He said: “There are types of incidents we can go to that we can manage, like bin fires or car fires, that perhaps don’t need a crew of five but the simple fact is the safe number of firefighters to carry out a rescue from a standard house fire is five. We’ve presented risk assessments that prove this categorically. Public safety will be compromised.”
But Leicestershire Fire and Rescue Service chief fire and rescue officer Dave Webb insisted the move would not compromise safety.
He said: “We’ve been operating, due to circumstances beyond our control, with four firefighters on a crew on each fire engine as a standard for about the last 12 months. During those 12 months we’ve been able to record, for the first time ever in Leicestershire Fire Service, zero accidental fire deaths and no significant injuries to any of our firefighters. The facts just do not back up the scaremongering going on around this issue.”