Planned cuts by police forces fall £500 million short of the savings needed, Whitehall's spending watchdog has warned.
The National Audit Office (NAO) said the 43 forces across England and Wales needed to make around £1.5 billion of savings, but around two-thirds of forces had shortfalls in their plans to cut costs last year.
Some forces had only planned for three years instead of four, while in others "the plan simply did not yield sufficient savings", the report said.
The warning comes after Her Majesty's Inspectorate of Constabulary (HMIC) found 17 forces had plans to make the necessary savings, while 26 had shortfalls amounting to about £500 million.
The NAO report added that the Home Office "believes that most of the savings gap has since been covered by forces' plans" but this cannot be confirmed until a further HMIC report is published later this year.
Home Secretary Theresa May's department also needs to do more to increase confidence that savings are being made in the right areas, the watchdog said, adding that business areas within the Home Office "have not fully considered efficiency and effectiveness when evaluating where cuts should be made".
The watchdog also said the Home Office needs to control transition costs "more strongly and explicitly" as it brings in the National Crime Agency (NCA) and the Disclosure and Barring Service (DBS), while phasing out the National Policing Improvement Agency (NPIA).
The report added: "There are risks that these programmes will fail to deliver the proposed benefits if the department does not manage them more effectively. The success of key change programmes is at risk because of poor continuity in personnel, insufficient resourcing and weak elements of financial and project management."
A Home Office spokeswoman said: "At a time of reducing budgets, we are working to ensure we get the best value from every pound spent on protecting the public. By improving efficiency, driving out waste, and increasing productivity, we can maintain a strong police service, a secure border and effective counter terrorism capabilities while delivering significant savings."
On the cuts to police budgets, she added: "The funding settlement is challenging but manageable and savings can be made whilst protecting frontline services. HMIC made clear last July that forces were putting plans in place to identify the necessary savings. We are confident the gap is closing and the HMIC will report later this year on the progress being made."