The Titanic actress and 20 other guests staying at the private Caribbean retreat in the British Virgin Islands escaped unharmed after lightening hit the mansion and set it ablaze.
Branson, who was staying at another property nearby with his wife and son, has told the British media the house has been ""completely destroyed"" by the fire.
Other guests who were forced to flee the burning building included Branson's 90-year-old mother, Eve, and his daughter Holly.
Branson built the Great House a year after buying Necker Island in 1981. It has become a retreat for a host of celebrities over the years - Robert De Niro and Steven Spielberg have both vacationed there.
The Virgin boss is planning to rebuild the property ""as soon as we can"".
He tells Britain's Daily Telegraph, ""There's a lot of damage and we won't be able to stick it back together again right away. It was a beautiful house... My office was based in the house and I have lost thousands of photographs, which is very sad.""
Branson was hoping to stage his daughter Holly's wedding to fiance Freddie Andrews at the property later this year (11).