Home Secretary John Reid's plans to split the Home Office in two could damage the UK's security, a think tank has warned.
Demos said Britain already risked being "overwhelmed" by the threat of terrorism, domestic unrest and the drugs trade because of a fragmented approach. And it suggested the solution was a national security "vision" - under the direct control of the Prime Minister.
Mr Reid wants to divide the mammoth Whitehall department he said was not "fit for purpose" into a Ministry of Justice and a Security Department.
The proposal is under discussion by Cabinet colleagues, but Tuesday's report warned it could worsen an already "overly fragmented approach to security". It blamed the lack of an overall plan for "infighting between departments, mixed messages from politicians and the duplication of resources".
The report, written by Demos lead security researcher Charlie Edwards, concluded: "Managing national security without a strategy is like running an orchestra without a musical score: a recipe for an ill-co-ordinated and out-of-tune response.
"A national security strategy is not a panacea for joined-up government and there are limitations to strategy: a strategy by no means renders national security invulnerable to threats. Nor does a strategy eradicate all risks.
"It would, however, enable the Government to communicate clearly concerning its ability and inability to safeguard national security. While there is no doubt that the Government understands the size and nature of these threats and hazards, the lack of an overarching strategy that places all these challenges into context with one another is lacking.
"In the absence of a clear framework, it has become increasingly difficult to prioritise policy at national level for the future, as the focus remains on managing the short term, often causing further problems down the road. Too often, the British Government has relied on individual departments and agencies - as well as occasional good luck - to win the fight, but such a piecemeal approach to security can only end in tragedy.
"Creating a national security strategy will not be an easy process, but it is the first step in understanding how the Government should respond to the dynamic and changing threats and hazards to the UK today and in the future."
The report suggested Parliament should debate the strategy and be updated annually on its implementation as well as scrutinising it through a select committee. And it said a single counter-terrorism budget and a separate fund for policy development could be needed.