"While tourists on all-inclusive holidays would have no need to worry, others should take more cash in euros with them in case the supply of drachma was not yet fully guaranteed throughout Greece," Markus Bruchmueller, a product manager for TUI Germany, said in an interview published on the group's website on Friday.

A TUI Germany spokeswoman said the advice came after its travel agents reported a surge in questions from nervous customers on what would happen if Greece were to exit the euro zone and reintroduce its old national currency.

"Since the elections at the start of May and the whole debate about Greece leaving the euro zone started up again, customers who are booked on trips to Greece, or even just thinking about it, are worried about what will happen to their money," spokeswoman Anja Braun told Reuters on Friday.

Tourism is a key source of income for Greece, accounting for about a fifth of gross domestic product, and helped narrow Greece's current account gap to 9.8 percent of GDP in 2011.

However, on Friday, the country's central bank said tourism income had dropped 15 percent in the first quarter, with fewer holidaymakers from Germany and Britain, Greece's biggest tourist markets.

Travel firms and airlines in Europe's largest economy have reported bookings down by a third in recent weeks, as scenes of violence from the capital Athens and a report of an attack on a man suspected of being German have prompted sun-seeking Germans to book holidays in Spain or Turkey instead.

Along with offering discounts, rival firms such as TUI Germany, Thomas Cook Germany (TCG.L) and Rewe have even clubbed together to take out joint adverts in the trade press, extolling the qualities of Greece.

"It's an important destination for us," TUI's Braun said. "There is no reason not to go to Greece. They are waiting with open arms for German tourists."

She said TUI Germany even held its annual sales event in Greece this year, meaning its travel agents could now tell customers first-hand what the situation was like.

For tour operators, Bruchmueller said in the interview, the return of the drachma would lead to contract renegotiations with hoteliers.

"But even after a reintroduction of the drachma, the euro would still be a basis for cooperation in the tourism industry," he said, adding that advance payments to hoteliers in Egypt and Tunisia, for example, are linked to the euro.

TUI Germany is part of TUI Travel (TT.L), Europe's largest tour operator. The group is controlled by TUI AG (TUIGn.DE).

Earlier this week, a Barclays economist warned that should Greece leave the euro zone, tourists could face food and fuel shortages, and that limits could be put on the amounts of money travellers would be allowed to take in and out of the country.

(Reporting by Victoria Bryan; Editing by Catherine Evans)

• A Permanent Economic Emergency