The heads of London’s Heathrow Airport and Philadelphia International Airport issued a joint call for relaxed travel restrictions between the United States and the United Kingdom.

Pandemic rules in the United States have barred non-citizens flying from the U.K., while British Prime Minister Boris Johnson’s government has imposed quarantine and COVID-19 testing requirements on travelers from the U.S.

In a letter to President Joe Biden and Johnson this week, Heathrow CEO John Holland-Kaye and PHL CEO Chellie Cameron argued that vaccination campaigns have advanced enough in both countries to be able to reopen air travel more broadly — before the G7 summit opens in Britain next month.

The two airport executives “urge both the U.S. and U.K. governments to take the decisive and necessary action to reopen U.S.-U.K. air travel — safely, securely and simultaneously — ahead of the G7 summit in June,” according to the letter.

The White House did not comment Tuesday.

A spokesperson for the U.K. government said: “The U.K. looks forward to working with the travel industry and the U.S. government to enable more travel between our two countries as conditions allow.”

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Typically, more than 20 million people a year travel between the U.S. and the U.K., the letter from the airport CEOs said. PHL is a transatlantic hub for American Airlines, and Heathrow is PHL’s busiest transatlantic route, with 455,000 passengers in 2019, according to the airport.

The U.S.-U.K. corridor is “an essential route for trade, travel and tourism,” said Jane Rosenberg, executive director of the British American Business Council of Greater Philadelphia.

But international travel has been limited since coronavirus lockdown orders went into effect in March 2020 during the Trump administration. After Biden took office in January, he also suspended entry into the country for non-citizens coming from the U.K. and many European countries, among others.

Last week, the U.K. unveiled a new ranking of countries and corresponding travel rules that begin May 17, and are denoted by color: green, amber, and red. The U.S. did not make it onto the least-restrictive “green” list. Instead, it landed on the list of amber countries, from which travelers must undergo a 10-day quarantine and a series of COVID-19 tests.

U.K. Transport Secretary Grant Shapps said that the list was “just a first step,” and that Britain would review it every three weeks.

Still, exclusions from the green list, limited to 12 countries and territories, drew criticism from trade groups such as the U.S. Travel Association.

“Putting the U.S. on amber status ignores the scientific data regarding increasing vaccination rates, lower infection rates and that the U.S. has the right strategies in place to mitigate risk,” the trade association’s CEO, Roger Dow, said in a statement May 7. “The U.S. needs to demonstrate leadership and come to the table with the U.K. and increase dialogue to allow for a reopening of travel with one of our most important international partners.”

As of Monday, 58% of U.S. adults, aged 18 and over, had received at least one vaccine dose, according to the Centers for Disease Control and Prevention. In the U.K., that figure is more than 67%, according to government data.

Cameron and Holland-Kaye said taking steps to reopen U.S.-U.K. travel now could also “drive the development of common international standards in air travel” when it comes to COVID-19.

PHL and Heathrow have also signed a memorandum of understanding to work together on COVID-19 recovery efforts. “This partnership will help us safely restart travel for our most important transatlantic trade corridor,” Cameron said in a statement Tuesday.