A planned programme to transform the radio system used by emergency services is "inherently high risk", a spending watchdog has warned.
The proposed new network for police, fire and ambulance services is not yet in use nationwide anywhere in the world, the National Audit Office (NAO) said.
The Government's chosen option to replace Airwave, the current communication service, is known as the Emergency Services Network (ESN).
Airwave has served emergency services effectively, averaging 99.9% availability since April 2010, according to the NAO.
Its report said setting up the new facility will cost around £1.2 billion to March 2020, but after that it will cost an estimated £500 less than Airwave per device per year.
The new network is expected to save money by using parts of EE's existing commercial 4G network.
International comparison work concluded that ESN is the most advanced programme in the world, with only one other country - South Korea - seeking to deploy a similar solution.
The NAO concluded that it is "the right direction strategically" - but said there are some "significant technical challenges" to overcome.
These were said to include working with EE to increase the coverage of its 4G network and developing handheld and vehicle mounted devices as none currently exist that would work on ESN.
It is currently projected that sufficient network coverage will be available by September 2017. The process of moving on to the new network is due to be finished in December 2019.
The report also said emergency services and other users of Airwave were concerned that ESN will not replicate all of Airwave's functionality, adding: "It is unclear, for example, whether the current specifications for ESN will meet the security needs of counter-terrorism and covert operatives."
NAO head Amyas Morse said: "The need to save money and get out of a difficult commercial relationship has led the Government to try and move to an approach that is not yet used nationwide anywhere in the world.
"The programme remains inherently high risk and while steps have been taken to manage these risks we are concerned that these are under-rated in the Home Office and elsewhere.
"The programme needs to put in place more independent testing and assurance regimes for its technical solution and urgently improve its approach to engaging with the emergency services."
Meg Hillier, chair of the Commons Public Accounts committee, said: "The Home Office cannot afford to get this wrong."
In 2011 the government set up a programme to look at options for replacing the Airwave service when the contracts end.
Airwave was sold to Motorola Solutions in February this year.
A spokesman for Motorola Solutions, which is the supplier of user services for the new ESN system, said: "We share ESN´s commitment and vision to providing emergency and public service agencies with the functionality and capabilities that a broadband-based network can offer.
"As the owners of the existing Airwave network, we believe we are best positioned to secure a smooth transition to ESN."
An EE spokesman said: "As the report suggests, Emergency Services Network is a state-of-the-art technology programme that will allow Britain's police, fire and ambulance services to benefit from a world-leading communications network.
"We're proud to be a part of this programme, and we're confident in delivering our commitments so that the lives of Britain's emergency services workers will be improved, not put at risk.
"Failing to replace the current, outdated systems will prevent Britain's emergency services from becoming safer, more efficient and more effective, and risks leaving them behind as technology advances around them."
A Home Office spokeswoman said: "We are developing the new Emergency Services Network because it will help keep people safe, providing the dedicated teams who work so hard to protect the public and save lives with the most advanced communications system of its kind anywhere in the world.
"It will ensure that police, fire and rescue and ambulance crews can do their jobs more effectively and efficiently, and will be a more capable and more flexible communications network than the existing Airwave system.
"As the National Audit Office's report has itself concluded, ESN is the right direction strategically for maximising these benefits.
"The timescale for ESN is deliberately ambitious because we want to maximise the benefits it will bring to the public and we have comprehensive risk management tools in place as well as the best possible expertise to design, build, test and roll out the new network."
Jack Dromey, former shadow home affairs minister, said: "The police and emergency services must be able to count on a communications system that works, including in a crisis.
"The Home Office are trying to get a new communications system on the cheap, covering but 70% of Britain as opposed to the 99% of Airwave.
"Ministers have been warned repeatedly that they will put at risk the safety and security of the police and the public if they proceed with a second-rate cheap alternative to Airwave. A failure at a time of crisis could have catastrophic consequences for life and limb."