Ministers should draw up plans to deal with a break-up of the eurozone "as a matter of urgency", a committee of MPs and peers has warned.
The joint committee on the government's National Security Strategy (NSS) said the full or partial collapse of the single currency was "plausible".
It said political unrest and a rise in economic migrant numbers could result.
"Long term security" is at the heart of foreign policy thinking, the government said in response.
The committee, whose members include ex-MI5 director general Baroness Manningham-Buller, said economic instability could leave the UK "unable to defend itself".
It added that governments across the EU could be forced to cut defence spending if the instability were to continue.
"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," it said.
A leading European Union official has urged private holders of Greek bonds to sign up to a vital debt swap deal ahead of a deadline later on Thursday.
Economic and Monetary Affairs Commissioner Olli Rehn said there would be no better offer, and the deal was vital for eurozone financial stability.
Greece needs at least 75% of bondholders to agree to take a 53.5% cut in the value of their holdings.
Greece needs the deal if it is to receive a second bailout.
The package from the European Union and International Monetary Fund would be worth 130bn euros ($171bn; £109bn).
Mr Rehn said: "It is important that all investors recognise that Europe has committed the maximum funds available to this voluntary debt exchange and that full participation is necessary for the Greek program to move forward."
Private investors have until 2000 GMT on Thursday to agree to the debt swap on the 206bn euros of Greek bonds they hold.
On Wednesday, the Institute of International Finance said that just under 40% of private holders of the outstanding Greek debt had agreed to it