People are increasingly putting off their summer holidays and booking autumn breaks instead due to uncertainty over international travel, tour operators say.
Thomas Cook said autumn bookings were up 50% on normal levels, while luxury tour operator Kuoni said customers were "holding out for October half term".
Greece and the Caribbean are among the most popular destinations.
But some fear prices will rise due to fewer countries being open to tourism.
International travel is due to resume from 17 May under the government's roadmap to reopening, with plans for a "traffic-light" system categorising different countries.
However, the list of countries that Britons can travel to will not be known until nearer 17 May and will be subject to change.
This has prompted a surge in autumn holiday bookings, with breaks in September and October accounting for 30% of Thomas Cook's live bookings for the year. That is up 50% on what the group normally sees in these months.
A spokesman said there was a "clear preference for September and October compared to what we normally expect".
He said many people had already booked domestic breaks for July and August and were looking for a "week of sun in the Med" later in the year.
"Devon is lovely, but Devon in October is not Devon in July," he added.
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According to Thomas Cook, Greece is the most popular destination for autumn breaks, with Rhodes the most in-demand island followed by Crete, Santorini and Mykonos.
The Canary Islands, a popular winter-sun destination, are the second most popular destination.
Kuoni did not share booking figures but said the majority of 2021 sales have been from October onwards. It added that customers were "saving and upgrading" for long-haul winter sun breaks.
"We've chatted to lots of customers who have written off any hope of getting away in the summer but are holding out for October half-term," the company said.
Its most popular destinations are the Maldives and the Caribbean, including Antigua, Barbados and Saint Lucia.
Other travel firms are seeing evidence of the trend in searches on their websites. British Airways Holidays said there was a lot of "pent-up demand" for foreign holidays and it has seen a spike in searches for trips to long-haul beach destinations this autumn.
Searches for December holidays have also seen a "notable increase", with New York proving popular.
Meanwhile travel deals website Travelzoo predicts September and October will be "big months for travel".
A survey of 2,000 of its members carried out last week found the majority said they plan to take a trip around that time.
However, the company warned that prices could increase if demand continues and there is a limited number of destinations open for travel.
"Right now we are seeing some good prices for September and October, but there's every indication it will start to prove more challenging for [package-holiday customers]," the company said.
Under the government's proposed traffic-light system, counties will be categorised as red, amber or green depending their infection rates, vaccination coverage and the prevalence of variants of concern.
Covid testing and quarantine requirements will vary from category to category. All passengers will have to take Covid tests before leaving and on returning - even from low-risk "green" countries.
There will also be a green "watchlist" which will identify countries that are most at risk of moving from green to amber.
Covid cases in some European countries remain high and vaccination programmes in the bloc have lagged behind the UK.
The Association of Independent Tour Operators said: "Would-be travellers are nervous without certainty. We need those government traffic lights to be confirmed to give us all confidence