Hospital staff had to administer electric shock treatment to the prime minister to regulate his heart before carrying out further tests.
A Downing Street spokesman said Blair was released after several hours of tests on Sunday, after he complained of heart palpitations.
He stressed the Prime Minister was now feeling fine.
"He was feeling under the weather earlier today and is now back in Downing Street," the spokesman said. "He is well and in good spirits."
The Downing Street spokesman said doctors regulated Blair's heart beat at a London hospital and the procedure was a success.
"It was established that he had an irregular heart beat," the spokesman said. "He was in hospital for four to five hours and he is now back in Number 10 (Downing Street)."
Blair underwent a procedure called cardio-version, where an electrical shock is delivered to the heart to return it to its normal rhythm.
Blair, 50, is a keep-fit enthusiast, regularly running on a treadmill and joining US President George Bush in the gym on a recent US trip.
The youngest prime minister since 1812, Blair has this year endured the toughest test of his six-year rule dealing with the Iraq war and his subsequent role in the naming of a weapons expert whose suicide battered the government's popularity.