NEW YORK -- The number of wealthy Americans applying for citizenship or residency in foreign countries has skyrocketed over the past three years as US billionaires, tech entrepreneurs and celebrities look to create a "plan B" for their families, reported major news portal Insider last week, based on interviews with multiple investment migration firms.

Latitude Residency & Citizenship, a company which guides high-net-worth investors around the world through the application process, was quoted as saying that US inquiries have increased 300 percent between 2019 and 2021.

Henley & Partners, one of the world's largest citizenship brokers, said sales to American nationals increased by 327 percent between 2019 and 2020 and an additional 10 percent in 2021.

According to Dominic Volek, head of private clients at Henley & Partners, there are "four C's" currently driving the investor citizenship industry: COVID-19, climate change, cryptocurrency, and conflict. The recent uptick in American applicants started during the Trump administration and escalated during pandemic lockdowns, he said.

"In the very strict lockdowns there was a point where if you only had an American passport, you could not enter Europe," said Volek. "I think that made a lot of particularly ultra high net worth individuals realize that they're potentially a little bit more fragile than they thought."

More than a dozen countries offer so-called "golden passports" and visas that allow affluent foreigners to receive citizenship or residency in exchange for investing in the country. The most expensive programs range from $1.1 million in Malta to $9.5 million in Austria, the Insider report said, citing Forbes.