A Greek and an EU flag fly over the Greece's finance ministry in Athens Thursday. (REUTERS/Yannis Behrakis)

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British tour operator Thomas Cook expects a surge in bookings to Greece if it leaves the euro zone as holidays to the Mediterranean nation would become better value for hard-pressed travellers.

"If Greece exits (the euro), for the tourism industry it could be very profitable," interim chief executive Sam Weihagen said after the company posted a steep first-half loss on Thursday.

"Most probably holidays to Greece will be more profitable for holidaymakers than they are today and places like Spain could lose competitiveness," he told reporters.

The 171-year-old travel group said summer bookings to Greece from Germany were down around 20% year-on-year but that bookings from elsewhere to the crisis-hit nation had held up.

Tourism is a vital source of income for Greece, accounting for about a fifth of gross domestic product. The outcome of an election next month will likely decide whether Greece remains in the euro.

Thomas Cook has been hit hard by tough trading conditions, especially in Britain where its core customer base of families with young children has been particularly affected by the economic downturn. It has also been hit by unrest in popular destinations such as Egypt, Tunisia and Morocco.

The world's oldest travel group continues to expect this year to be challenging given the economic backdrop and difficult trading environment, particularly in North America and France. It said its full-year performance would be dependent on how well it performs in the late bookings market.

Thomas Cook said year-on-year U.K. bookings were up 5% over the last four weeks but that demand from British corporate clients for Olympics packages had been weaker than expected.

"U.K. corporations are less interested in buying our Olympics packages, maybe their profits are stretched or they think they shouldn't be buying these," said Weihagen. "We're transferring a lot of them to regular customers and there is huge demand."