Foreign Secretary William Hague has warned of the problems facing Greece if it decides to abandon the euro.
"They have to be prepared to take action to stop euros leaving the country, maybe have border controls," he told the Andrew Marr show.
Eurozone finance ministers are due to discuss a 130bn euro ($171bn; £108bn) bailout package for Greece on Monday.
"They don't have the old currency sitting in the vaults ready to distribute," he said.
"It's not straightforward to leave the euro. It was built without exits."
Shadow chancellor Ed Balls criticised the steps that Germany was taking to support the eurozone.
"For Greece to stay in the euro... means a long and protracted period of slow growth, high unemployment, at a time when the German economy is not really willing to pull its weight - the Germans aren't doing what needs to be done," he told Andrew Marr.
"A messy Greek default, which raises questions that the markets will ask Spain and Italy next, at a time when Germany and the European Central Bank are not able to face up to the common obligations you need in a single currency: that could lead to a crisis in the eurozone, which would be very dangerous indeed."
Mr Balls also ruled out taking the UK into the euro in his political lifetime.
Mr Hague said that greater integration would be needed to make the euro work.
"It does require closer fiscal integration, it does require closer political integration - it can't work without that," he said.
"When Germany is in a currency like that it imposes on the others Germanic discipline if they are going to be able to stay in it for the long term."
Asked about whether he would be happy for Britain to contribute to an IMF bailout of Greece, Mr Balls said that should only happen if it was not a substitute for funding that should come from the European Central Bank (ECB).
"If the IMF funding is supposed to go in to substitute for the funding which should come from the ECB, because Germany says we're not willing to fund the ECB, that would not only be the wrong thing it would be completely counterproductive," he said.
"I don't think Germany has faced up to the reality it's in a single currency and there are collective obligations in a single currency. They joined it, they've now got to make the best of it, and that's not happening at the moment."