British citizens who are caught with cannabis could find themselves banned from the US for life, even if they are smoking up in a state where pot is legal.

Charlotte Slocombe, senior partner at Fragomen, a London-based legal firm that deals with international immigration laws, has warned British tourists and green card holders that they can be arrested — and even deported — from the US if they are caught with weed. As more and more US states have legalized cannabis, immigration attorneys have seen a marked increase in the number of foreign citizens being deported or denied entry to the country over minor weed use.

“Even if you are a foreigner in a state where it’s legal, it’s still illegal for you federally,” Slocombe explained to The Guardian . “If, for example, you buy cannabis from a legal dispensary and that dispensary asks for your passport details as ID that information might be discoverable. It could then trigger, as can admission of drug taking, immigration issues because immigration is covered by federal law not state law.”

“This is how people get caught even though they think they are doing something which is now legal in that state,” she continued . “Equally that would include green card holders and those who hold visas to live and work in the US. Even for an American it is federally illegal, but because they are not subject to US federal immigration laws they would not be as vulnerable.”

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As another example, Slocombe explained that Americans who were caught smoking weed at a party could not be arrested by state cops if that state had legalized adult-use. But if foreign tourists or other green card holders were caught smoking weed at that same party, state police could arrest them and turn them over to border police. From there, immigration authorities could then deport them and ban them from entering the US for life.

This issue came to a head last year when Canada legalized adult-use. US Customs and Border Protection (CBP) officials reiterated that cannabis remains illegal under federal law, and that any Canadian who admitted to smoking weed even once could be banned from the US for life . This summer, one Canadian woman was banned from the country for trying to bring a bottle of CBD oil — which is completely legal in both Canada and the US — across the border.

America's draconian immigration laws do not only apply to cannabis consumers, but to employees of the cannabis industry as well. CBP officials initially said that any employee of any Canadian cannabis company would also be banned from entering the US, but eventually reversed their decision . Still, Slocombe told The Guardian that she has handled many cases where customs officials denied visas to foreign investors in the weed industry. And, in another case, a farmer's visa was challenged merely because he sold part of his land to a cannabis producer.