Ministers should draw up plans to deal with a collapse of the eurozone "as a matter of urgency", a high-powered committee of MPs and peers warned on Thursday morning
The joint committee on the Government's National Security Strategy (NSS) described the full or partial collapse of the single currency area as a "plausible scenario".
The consequences, it said, could include the outbreak of "domestic social or political unrest" and a surge of economic migrants within the European Union.
Further international economic instability could even leave the UK "unable to afford to defend itself", with governments across the EU forced to slash defence spending, it added.
In a wide-ranging report, the committee - whose members include former MI5 director general Baroness Manningham-Buller - said Britain may have to re-think its relationship with the United States, as Washington realigned its strategic priorities.
While the committee welcomed the Government's decision to publish the NSS alongside the 2010 Strategic Defence and Security Review, it said that it had so far failed to produce "a clear over-arching strategy".
It dismissed the assertion in the NSS that the UK did not face any loss of power or influence as global economic activity shifted to Asia and Latin America as "wholly unrealistic".
It said the failure of the National Security Council (NSC), chaired by the Prime Minister, to consider such issues as the potential impact of Scottish independence or a fresh economic crisis showed its oversight of security issues was "not sufficiently broad or strategic".
"International economic problems could lead to our allies having to make considerable cuts to their defence spending, and to an increase in economic migrants between EU member states, and to domestic social or political unrest," the committee said.
"We believe that, even in 2010, the potential threat to UK security from a full, or partial, collapse of the eurozone was one of the plausible scenarios which a prudent NSRA (national security risk assessment) should have examined.
"We call on the NSC to address the potential impacts on the UK and Nato (and how the Government would respond) were this to happen, as a matter of urgency."
The committee said that the Government also needed to "reflect deeply" on the implications for Britain of the policy shifts taking place in the US as Washington increasingly turns its focus away from Europe.
"It raises fundamental questions if our pre-eminent defence and security relationship is with an ally who has interests which are increasingly divergent from our own," it said.
"The Government needs to decide if the UK will continue to be as involved in US military action as we have been in the past if the US focuses on Asia-Pacific."
The committee said that in an era of "diminished resources", the UK would have to accept a more "partnership-dependent" role in world affairs.
"We believe it is totally unrealistic not to expect any diminution in the UK's power and influence in the medium and long term," it said.
The committee said there was no evidence the NSS had guided Government policy when it came to dealing with issues such as the Arab Spring.
At the same time, it complained that there was a tendency for the NSC to be side-tracked into short-term crisis-management of issues like Libya, when it should be focusing on risks emerging "over the horizon".
"A good strategy is realistic, is clear on the big questions, and guides choices. This one does not," said the committee chairman, former foreign secretary Margaret Beckett.
"We need a public debate on the sort of country we want the UK to be in future and whether our ambitions are realistic, given how much we are prepared to spend."
A Government spokesman said: "A strategy for Britain's long term security and prosperity is at the heart of the Government's approach to foreign policy.
"We remain vigilant and regularly take stock of the changing global environment and threats to our security, as well as opportunities for our country to make the most of all its assets and advantages in a networked world."