Chancellor Rishi Sunak held a US green card - allowing permanent residence in that country - while he was chancellor, his spokeswoman has said.
He returned it in October last year, ahead of his first American trip as a UK government minister.
The news comes as Mr Sunak faces scrutiny over the revelation that his wife is not liable for UK taxes on her overseas earnings.
He has accused his critics of running a "smear" campaign against his family.
But the Liberal Democrats are calling for Mr Sunak, who became chancellor in early 2020, to resign, with leader Sir Ed Davey saying: "Never mind a green card; it's time to give Rishi Sunak the red card."
As stipulated in the rules, Mr Sunak filed US tax returns while he held his green card, his spokeswoman said, "but specifically as a non-resident, in full compliance with the law".
She added: "As required under US law and as advised, he continued to use his green card for travel purposes. Upon his first trip to the US in a government capacity as chancellor, he discussed the appropriate course of action with the US authorities.
"At that point it was considered best to return his green card, which he did immediately."
Asked about Mr Sunak's green card at a Downing Street press conference, Prime Minister Boris Johnson said: "As I understand it, the chancellor has done absolutely everything he was required to do."
For Labour, shadow Treasury minister Pat McFadden said there were further questions for Mr Sunak to answer. "Why did he keep a green card for so long while being a UK MP?" he asked.
"Was there any tax advantage in doing so? And why did he give this up after holding it for so long?"
The chancellor studied for a master of business administration (MBA) post-graduate degree at Stanford University, California, from 2004 to 2006, where he met fellow student Akshata Murty. The couple married in 2009 and have two daughters.
Ms Murty, the daughter of an Indian billionaire, has revealed she has "non-dom" status, meaning she does not pay UK tax on overseas earning, which the BBC estimates saves her more than £2m a year.
Mr Sunak has said it is unfair to attack his wife when she is a "private citizen", adding: "I'm an elected politician. So I know what I signed up for."
"To smear my wife to get at me is awful," he added.