Britain's emergency services face a "potentially catastrophic" six-month gap without a crucial communications system, a Commons report warns.
MPs raised the alarm over the impact of allowing a network used by all 105 police, fire and rescue and ambulance services in the country to be taken out of service early.
The Public Accounts Committee (PAC) said the need for blue-lights teams to communicate in the event of an emergency, including a terrorist attack, is an essential part of keeping the public safe.
Meg Hillier, chairwoman of the committee, said: "The potential consequences of a six-month gap in emergency service communications are unthinkable.
"Government needs to tackle this now or the result will be quite simply a tragedy in waiting."
Responding to the report, ministers insisted there will be no gap in the emergency services' communications provision.
The PAC's assessment centres on a £1.2 billion project to replace the current Airwave system with a new Emergency Services Network (ESN) which will use parts of EE's 4G network.
Earlier this year, the committee concluded that it was unlikely that the December 2019 target date for delivering the upgrade would be met.
The period for transitioning on to the new system now runs to September 2020, nine months later than originally planned, according to the committee's latest paper.
It has also come to light that extending the Airwave contracts, the Home Office's "sole mitigation" against delay in putting the new system in place, might not be possible, the report warns.
It says that in January Airwave owner Motorola Solutions told the committee that Vodafone, a key supplier to Airwave, will from March 2020 stop providing an important piece of infrastructure that Airwave requires to function, essentially turning it off.
This raised the possibility that emergency services may not be able to communicate with each other after March 2020 until transition to ESN is complete in September 2020, the committee said.
The MPs called on the Home Office to engage urgently with the two companies on the options for resolving the Airwave issue.
Vodafone UK, which is not involved in delivering the new system, said it has been in detailed discussions with the Home Office and Motorola Solutions since it became clear that the ESN project could be delayed.
A Vodafone UK spokesman said: "We are actively working with all relevant parties to find an alternative solution to the old legacy fixed network used to link Airwave masts and to ensure continuation of service if the roll-out of ESN misses its target date."
Motorola Solutions said: "We are committed to a smooth transition of the emergency services from Airwave to the Emergency Services Network (ESN). Airwave is not being switched off early.
"We identified the need to upgrade the legacy core transmission network provided by Vodafone to ensure that Airwave can continue beyond its original operational end-date of March 2020.
"We are working with the Home Office and Vodafone to identify viable technical options to extend the service and will be proposing our recommendation in June 2017 to the Home Office.
"None of this impacts our Lot 2 delivery of the ESN for which we are on track."
Brandon Lewis, minister for policing and the fire service, said: "We are clear that we won't take any risks with public safety and there will be no gap in the emergency services' communications provision.
"The existing Airwave system will continue until transition on to the Emergency Services Network is completed.
"Keeping people safe is our priority, which is why we are delivering the world-leading ESN to support our police, fire and rescue and ambulance crews who work so hard protecting the public and saving lives."