The idea was branded “totally needless” and “high-risk” as the North Yorkshire police and crime panel rejected the case for the overhaul.
North Yorkshire County Council and York Council have already voiced their opposition although the Fire Brigades Union has come out in favour of the change.
Mrs Mulligan has proposed taking responsibility for the fire service as well as the police to help improve collaboration between the two services and save money.
Her proposal suggests £6.6m could be saved over 10 years if her office took the place of the North Yorkshire Fire and Rescue Authority which currently holds the fire service to account.
The police and crime panel, which holds Mrs Mulligan to account in her current role, joined York Council and North Yorkshire in recommending she should instead be given a seat on the fire authority.
Panel vice-chairman Peter Wilkinson said: “Such a drastic change in governance of the fire service is totally needless in North Yorkshire.
“We understand that there are parts of the country with under-performing fire services where this ‘governance model’ may appeal.
“However, we have an effective fire authority with strong leadership, which has taken forward a number of good collaborative initiatives with other partners, such as the health sector.
“Taking a very narrow focus on pushing police and fire together risks excluding such valuable partnerships and reducing the range of collaborative opportunities that would benefit communities.”
The proposal set out by Mrs Mulligan argued making her accountable for the fire service would “make transformational change more likely” and “bring more significant financial benefits that could be re-invested in frontline services”.
Coun Wilkinson said: “The panel agrees with the general principle of improving collaboration for the benefit of communities and where it also achieves savings that can be re-invested into frontline services.
“But the commissioner’s proposal goes too far. She is asking the public – and we as partners – to agree to a proposal which is very high-risk, lacking in any detailed assessment of what it would achieve and which, critically, cannot be reversed if things go wrong.”
Under the option preferred by the police and crime panel, North Yorkshire County Council and York Council, Mrs Mulligan would become the 17th member of the fire authority rather than replacing the body.
She would also sit on “collaboration sub-committee” focused on the police and fire service working more closely.
North Yorkshire County Council leader Carl Les has described this as a “stepped approach” allowing for more radical reforms later if required.
But in her report setting out the case for taking over the fire service, Mrs Mulligan argued that merely taking a seat on the current fire authority was “unlikely to drive a significant change in the pace or scale of collaboration” and would not “deliver significant savings”.
Mrs Mulligan is conducting a survey of public views on the propoals at www.northyorkshire-pcc.gov.uk/workingbettertogether/.