Travellers and tour operators were scrambling to react on Wednesday after Finland announced it would recommend people avoid travel to nine countries, and self-isolate if they arrive from those countries.
Included in the list was Greece, a popular destination with Finnish package tourists since restrictions were lifted in mid-July.
Since then arrivals from Greece have not been asked to self-isolate for a fortnight, but that will now change.
Tour operators have now cancelled trips to Greece until at least 7 September. The new restrictions come into force on Monday 24 August, but TUI has already cancelled departures scheduled for this week.
"We have flown three flights a week to Greece since mid-July but now we have to cancel them until further notice," said TUI's communications director Laura Aaltonen. "The situation will be followed over the next couple of weeks. Finnish travellers in Greece can continue their holidays as normal."
Customers can choose to either have their holiday dates changed, or to take a full refund. Operators reported their customer service channels were busy on Wednesday.
TUI reports on its website that customers should only contact them if they want to change dates. Otherwise refunds will be paid automatically.
Aurinkomatkat, meanwhile, has cancelled charter flights to Crete and Rhodes until 9 October, as well as flights to Madeira, the Canary islands and Turkey's Antalya coast.
The company said it would look at the situation from 10 October onwards nearer the time.