A single police force might not bring about projected savings in the expected timescale, a committee of MSPs has noted.
Holyrood's Justice Committee has now sought clarification from ministers about the impact that civilian redundancies arising from the Police and Fire Reform Bill will have on the duties of frontline officers.
The committee's request for clarity comes following weeks of evidence from witnesses who raised "concerns regarding the ability to achieve the projected savings".
Justice Secretary Kenny MacAskill has said the Bill will deliver estimated "efficiency savings" of £1.7bn over 15 years.
However, the committee's stage one report, released on Wednesday, featured evidence to the contrary from Chief Constable Kevin Smith, president of the Association of Chief Police Officers in Scotland.
He said that both his "personal and professional view was that the savings set out in the Bill will not be achieved in the coming years".
The report also features the views of councillor Barbara Grant, community safety spokesperson of Cosla (Convention of Scottish Local Authorities), who said "cops will have to take up the slack" if support staff are made redundant.
She said: "We really do not want to pay police, who should be out there doing the business on the street, to sit behind a desk and pick up a phone."
Her concerns were echoed by Dave Watson, from union Unison Scotland, who said the chief officer "will simply have to take officers off the streets to do civilian jobs".
The MSPs also called for the appointment of the chief constable of the new single force and the chief fire officer of the unified Fire and Rescue Service to be made as soon as possible.
Convener Christine Grahame said: "Our role in scrutinising this Bill is to make sure that the new national police and fire and rescue forces can actually deliver the services effectively through Scotland. The overwhelming majority of the committee believe that they can.
"However, what came over loud and clear from the vast majority of witnesses from the services was the need to appoint a chief constable and fire officer as soon as possible.
"Day one of the new national service is April 1 next year and an appointment date of December 2012 will not give enough time for the new heads to prepare for it."
Liberal Democrat justice spokeswoman Alison McInnes was the only committee member who did not agree with the general principles of the Bill.
She said: "This Bill represents the largest restructuring of our emergency services in a generation, yet there are a staggering number of questions still left unanswered.
"The committee has heard serious concerns from several witnesses that the business case that the government has put forward is flimsy at best.
"Worse, there is still no clarity about how local services will be protected once the new services are in place.
"For the sake of a ministerial power grab, the government are willing to risk the loss of frontline police officers, put our retained fire stations under threat and destroy local accountability.
"The Scottish Liberal Democrats have opposed the creation of single, national police and fire services from the outset. Having heard the evidence presented to committee on how they will operate, I am more convinced than ever that the government must rethink these flawed plans."