Delays in introducing a new radio system for the emergency services may cost taxpayers £475 million a year, MPs have warned.
The Commons Public Accounts Committee (PAC) has reported that the Emergency Services Network (ESN), which will replace the current Airwave system, could require more testing than the start date of December 2019 allows for.
The financial watchdog said postponing the introduction for a year could cost hundreds of millions of pounds, and that the Home Office has not budgeted for an extended transition period or put in place detailed contingency arrangements to manage this risk of delay for the police, fire and ambulance services.
The committee warns that good communications "can make the difference between life and death for both emergency services personnel and the public".
The PAC has called on the Home Office to put in place adequate independent testing of the ESN technology to ensure it "works under pressure in a live environment" and also address the "real security concerns" about communications on the London Underground and other tunnel transport systems.
The MPs have also raised concern that the Home Office "did not manage to maintain competitive pressure" during the letting of the two main ESN contracts.
Labour MP and PAC chairwoman Meg Hillier said: "The stakes in this programme are extremely high.
"It is absolutely right that emergency services will not commit to using ESN in potentially life-or-death situations until they are convinced it works.
"Questions continue to hang over the technology, not least how it will operate on underground rail systems in London and elsewhere - high-risk environments that present unique challenges in emergencies.
"These must be addressed urgently.
"It is encouraging that the head of the ESN programme has remained in post since 2011, providing a degree of stability absent from some high-profile projects our committee has examined.
"However, we are disappointed that detailed contingency plans have not been budgeted for or drawn up in the event that, as now seems likely, implementation over-runs.
"It is critical for public safety and achieving value for money that the Government has a firm grasp of the implications of delays in its timetable and a costed plan to tackle them.
"We will expect it to demonstrate real progress in this area when it reports back to us later this year."
The report states: "It seems unlikely that the ambitious target date for delivering ESN will be met.
"Any further compression of the timetable will increase the risk to successful delivery of this critical programme.
"The National Audit Office estimated that the programme was between five and ten months behind target and emergency services representatives are less than 50% confident that ESN will be delivered on time.
"Despite the prospect of delay the department has not budgeted for an extended transition period or put in place detailed contingency arrangements to manage this risk.
"The current Airwave contracts expire in December 2019 and the only contingency if ESN is not functional by then is to extend them, which would cost an estimated £475 million for a year's delay nationwide.
"The Department has negotiated a fixed price with Airwave's new owner, Motorola, to extend Airwave if needed on a regional and monthly basis but detailed contingency plans to manage any such extensions have not been prepared."
A Home Office spokesman said: "The new Emergency Services Network (ESN) will give the dedicated professionals who work so hard protecting the public and saving lives the most advanced communications system of its kind.
"Police, fire and rescue and ambulance crews will be able to do their work more effectively with ESN and the new system will deliver significant savings for the taxpayer.
"The timescales are ambitious because we want to get the most from technology that will help save lives but we are clear that no risks will be taken with public safety and the existing Airwave system will continue until transition on to ESN is completed."