Holidaymakers heading to Greece are being urged not to panic over their plans, but to take sufficient cash.
Consumer groups issued the warning as the Treasury said it is "stepping up" preparations for a Greek exit from the eurozone.
The country's central bank warned that such a move would lead to a deep recession with higher prices and thousands of people losing their jobs.
Bob Atkinson, a travel expert at website MoneySuperMarket.com, said: "Our recommendation is to have at least four to five days' worth of euros cash on you for the entire time you are there.
"The important thing is to have a cash flow," he said, adding that travel insurance policies often only cover a small amount of cash and travellers should therefore try to store their money in a hotel deposit box.
Andrew Brown, a spokesman for Post Office Travel Money, said there was no need to panic, as it would take 18 months for the drachma currency to be reintroduced.
It came as Whitehall officials made clear that all steps were being taken for the possibility of so-called "Grexit", which would pose serious economic risks to the UK economy, but declined to say what the contingency plans were.
The Government's planning work is being led by the Treasury in liaison with "the relevant actors" such as the Bank of England.
Chancellor George Osborne has told MPs "people should not underestimate" the damage a Grexit would do to financial confidence.
David Cameron's spokeswoman explained: "This is about making sure we can be as prepared as we can be in the event that this happens.
"The potential default or exit of Greece does present some serious economic risks. So alongside having contingency plans in place, it means ensuring that we have an economy that is growing, that our public finances are in a good order," she added.
The British Chambers of Commerce warned market upheavals caused by "a messy Grexit" could hit many UK businesses and called on central banks and governments to take action to limit disruption "through all means possible".
As crowds gathered outside the parliament in Athens, the high stakes stand-off between the Greek Government and the country's creditors in Europe and America continued.
Athens says it needs £5bn in bail-out funds to make a payment to the International Monetary Fund by the end of the month.
The Bank of Greece said agreement with the country's creditors was "a historical imperative that we cannot afford to ignore", warning failure would lead to country's exit from the euro area and, most likely, from the European Union".
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