The 34-year-old sailor crossed a virtual finish line drawn between the island of Ushant off France's northwest tip and Lizard Point in southwest England at 0145 GMT, comfortably beating the previous record set by compatriot Thomas Coville by six days and 10 hours.
The race time was announced by an observer from the World Sailing Speed Council but will be subject to checks of the boat's black box and its GPS data before final confirmation.
Father-of-two Gabart becomes just the fourth title-holder for a world record of sailing the globe solo.
Since the record was first set in 2004, some 30 days have been shaved off.
She remained undefeated until 2016 when Coville set a new record of 49 days and three hours which many predicted would be difficult to topple.
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