A new 999 radio system for Britain's emergency services is likely to be at least £3 billion over budget, a spending watchdog has said.
And the "critical" overhaul of the communications network is likely to be hit by further delays - and potentially even higher costs, the National Audit Office has warned.
The Home Office has already pushed back the planned switch off of the existing Airwave system by three years, to 2022.
But the NAO raised doubts over whether the replacement for Airwave, the Emergency Services Network (ESN), will be ready by the new target date.
Key technology for ESN is not yet proven in "real-world conditions", the NAO said.
It concluded that the Home Office's management of the programme has led to delays, increased costs and poor value for taxpayers.
Sir Amyas Morse, head of the NAO, said: "The success of the Emergency Services Network is critical to the day-to-day operations of our emergency services that keep us all safe.
"The Home Office needs a comprehensive plan with a realistic timetable that properly considers risks and uncertainties.
"It has already been through one costly reset and is in danger of needing another unless it gets its house in order."
Since 2000, emergency services have communicated using a dedicated digital radio network known as Airwave.
It is used by all 107 police, fire and ambulance services in England, Scotland and Wales, for communications between control rooms and teams in the field, as well as 363 other organisations such as local authorities and train operating companies.
In 2011, the Home Office decided to replace Airwave with ESN.
The new service is intended to match Airwave in all respects, allow users to take advantage of high-speed mobile data and save money by sharing an existing commercial 4G network.
It had been expected that emergency services would start using ESN in September 2017, with the transition being completed at the end of this year.
But the NAO said that by 2017, the Home Office realised its plan was not achievable.
Last year the department announced a "reset" of its approach, based on a phased introduction of ESN services, rather than launching the whole programme at once.
The new plan is for Airwave to be switched off in December 2022.
But the NAO's report said: "There are still significant risks and, based on past performance, it seems unlikely that ESN can be delivered by the target date of 2022."
The Home Office now predicts that ESN will cost £9.3 billion, £3.1 billion more than the sum forecast in 2015.
Of this rise, £1.4 billion is the cost of extending Airwave, £0.5 billion is an increase in contingency and the rest of the programme is now forecast to cost £1.2 billion more.
The NAO described the revised forecast costs as "highly uncertain".
It concluded: "To date, the Home Office's management of this critical programme has represented poor value for money."
Meg Hillier, chairwoman of the Public Accounts Committee, said: "The Emergency Services Network is to be used by police, firefighters and ambulance crews for the communications they need to do their life-saving jobs.
"The Home Office must take an urgent and honest examination of its ability to deliver to its new schedule for this critical project."
A Home Office spokeswoman said ESN is "on track to deliver an ambitious, world-leading digital communications network for the emergency services by 2022, resulting in savings of £200 million a year".