Serious question-marks about who would be in charge in the event of a national health emergency are hanging over the UK, MPs have warned.
A report from the Commons' communities and local government committee said the transferral of responsibility for health and wellbeing to local authorities, which takes place on April 1st, was creating "unacceptable" confusion.
It blamed the government for not setting out "clearly and unambiguously" where the lines of responsibility lie.
In the event of a health emergency rapid provision of screening and immunisation would become critical.
Delays seem inevitable amid ongoing uncertainty over who would be responsible.
"Without clarity there is only confusion, and a health emergency is no time for muddle," committee chair Clive Betts said.
"The government must set out unambiguously the lines of responsibility, and it must do so now as a matter of urgency. These arrangements need to be clear and in place on day one, April 1st. Anything else is unacceptable."
The Health Protection Agency suggestions "disruption for communities on a large scale" could be triggered by "emergencies, outbreaks of disease and chemical incidents". It adds: "Disease outbreaks and chemical incidents can develop very rapidly - so preparation and emergency planning are essential components in minimising the impact on the public."
The national outbreak scenario reflects the "real confusion" which the NHS' new health and wellbeing boards are now facing, MPs noted.
They said the government needed to clarify lines of accountability, underlining the hostility and frustration which the coalition's controversial NHS reforms continue to provoke.
The NHS commissioning board, which sits at the top of the pyramid, lacks local accountability and its status in relation to the health and wellbeing boards is unclear, MPs explained.
Local services are to be commissioned by clinical commissioning groups. These are to be held accountable but councillors are being prevented from sitting on them.
"The purpose of localism is not only to devolve decision making to a local level, but to make it accountable to local people," Betts added.
"With these changes it is clear that there is a shift of power and money from the Whitehall to local government and I welcome that. But the new arrangements are complex and responsibilities are shared across several bodies. The result is that lines of local accountability are fragmented and blurred."