Chancellor George Osborne has issued a last ditch appeal to the Greek government to do a deal with its creditors to prevent it falling out of the euro.
After a meeting in Luxembourg of eurozone finance ministers failed to make a breakthrough, Mr Osborne said people should be "prepared for the worst".
He said that Treasury officials would now go on to complete preparations to protect the British economy from the effects of a disorderly "Grexit" by Athens from the single currency.
"We have entered the eleventh hour of this Greek crisis, and we urge the Greek government to do a deal before it is too late. We hope for the best, but we now must be prepared for the worst," Mr Osborne said.
"In the United Kingdom we've taken the measures to increase our economic security so we can deal with risks like this from abroad. And clearly now we must go on and complete that plan."
The Chancellor is in Luxembourg for a full meeting of EU finance ministers following yesterday's eurozone group meeting.
The focus will now shift to Monday's emergency summit meeting in Brussels of the eurozone leaders which may offer the last chance of an agreement.