Olympic swimming superstar Michael Phelps has been arrested in his home state of Maryland and charged with driving under the influence.
Maryland Transportation Authority Police say Phelps was pulled over in the early hours on Tuesday after his white Land Rover was clocked at 84 miles (135 kilometres) per hour in a 45 mph zone in Baltimore's Fort McHenry Tunnel.
"He was unable to perform satisfactorily a series of standard field sobriety tests," it added in a statement distributed to local media.
The 29-year-old has been charged with driving under the influence (DUI), excessive speed and crossing double lane lines, it added.
On his @MichaelPhelps Twitter account, the most decorated Olympian of all time expressed remorse.
"I understand the severity of my actions and take full responsibility," he told his 1.6 million followers.
"I know these words may not mean much right now but I am deeply sorry to everyone I have let down."
Phelps, with 18 Olympic gold medals to his name, retired from competitive swimming after the 2012 Olympics in London, but launched a comeback earlier this year, possibly looking at one more Olympic run.
Last month, he made a dazzling return to international competition when he picked up three gold medals at the Pan Pacific Championships in Australia.
His strong showing - under the guidance of his longtime coach Bob Bowman - bolstered hopes that he might compete for the United States at the 2016 Olympics in Rio de Janeiro.
Ten years ago, Phelps pleaded guilty to driving while impaired in rural Maryland.
He was sentenced to 18 months' probation and fined $US250 ($A270) for what he later described as an "isolated incident."
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