The 'Gimme More' singer, who fell out with Paris at Britney's 26th birthday celebrations ,sent the hotel heiress a letter warning her she has an X-rated tape of Paris.
In the letter, she tells Paris - whose 2004 home-made sex tape 'One Night in Paris' was originally posted on the internet by ex-boyfriend Rick Salomon - the footage will be made public unless she treats her friends better.
A source said: "Britney and her pals wrote to Paris and told her they are hearing rumours of a new sex-tape scandal.
"The letter also claims the tape shows footage of Paris locking lips with one of Britney's female friends at a party.
"It warns Paris that if she continues being rude to people, the footage will be leaked online."
The note ended with Britney and her friends telling Paris they look forward to hearing back from her.
The pair are said to have fallen out at Britney's birthday bash after Paris snubbed her.
A source revealed to Britain's Daily Star newspaper: "It's obvious Britney and her bratty pals are being silly. Paris laughed when she got the letter, supposedly from Britney and said it was 'c**p.' "