American tourists may be allowed to travel to Britain next month as part of a plan to gradually reopen the U.K.’s borders, according to the head of London’s Heathrow airport.

Progress on COVID-19 vaccinations in the two countries could allow the U.S. to be included on the so-called Green List of countries from which people will be able to travel to Britain without quarantine, Heathrow Chief Executive Officer John Holland-Kaye said in an interview Thursday.

While it wouldn’t necessarily be reciprocal, the move would make it easier to travel to the U.K. from the U.S. than from some parts of the European Union, where coronavirus shots haven’t been rolled out as quickly.

Even a limited restart of U.S. services would come as a huge boost for Heathrow, which relies on the market for 20% of its traffic. The airport, Europe’s busiest prior to the pandemic, said passenger numbers remained stuck at just 9% of usual levels in the first quarter as current curbs limit operations at carriers such as British Airways and Virgin Atlantic Airways Ltd.

Locations also likely to feature on the U.K. Green List include Singapore, Hong Kong, Australia, New Zealand, Israel and Iceland, plus some Caribbean destinations, the CEO said. Heathrow has been in constant communication with the government over the impact of the pandemic as well as plans for a resumption in travel.

Holland-Kaye said he expects Prime Minister Boris Johnson’s government to publish the country list as soon as the end of next week.

The government has targeted a reopening of travel as soon as May 17, though the impact won’t be clear to airlines until criteria for how countries are divided between the three-category system are released. Arrivals from places marked green will still require a minimum of two COVID-19 tests, but those listed as amber or red will additionally face quarantines.

Some short-haul leisure destinations could be green-listed, but most likely only to island sunspots such as Spain’s Balearics and Canaries, Holland-Kaye said. Mainland Spain and Greece may not feature until June and July respectively.

The U.K. will confirm by early next month if international travel can resume on May 17, and which countries will fall into each category, a Department for Transport spokesman said in an email.

While a full deal with the U.S. might be difficult to reach in time for the mid-May target, a unilateral U.K. reopening could allow Americans to fly into the country for business and leisure trips, Holland-Kaye said. In this scenario, Brits seeking to go to the U.S. would have to wait for the Biden administration to reciprocate.

Bloomberg reported last week on bilateral talks between the U.S. and U.K. transport secretaries on a potential travel corridor.

As with the U.S., restrictions in Australia and New Zealand would limit any significant bump-up in air travel until the measures were reciprocal. Both countries are currently closed to all except residents and citizens.

While the U.S. has restricted entry from European countries including the U.K., Americans are allowed leave and return freely. The policy has led to a mini-boom in pandemic travel to Cancun in Mexico.

The European Union, which is behind the U.S. and U.K. in vaccinations, is also rushing to get a system in place that will allow for cross-border travel.

On Thursday, EasyJet CEO Johan Lundgren called on the U.K. to put most of Europe on the Green List, saying studies show it would be safe. In an Aviation Club U.K. webinar, the low-cost carrier reiterated its plea for the government to publish the standards that will determine the traffic-light tiers and name the countries on the green list.

Heathrow said earlier that it may attract only 13 million passengers this year if travel markets aren’t reopened, increasing to 36 million in a best-case scenario. The hub lured 81 travelers in 2019, a figure that fell to 22 million last year.