The 29-year-old singer - who has two sons, Sean Preston, five, and Jayden James, four, with second husband Kevin Federline - got secretly engaged to long-term boyfriend Jason Trawick when the 39-year-old talent manager popped the question by writing the pop star a romantic poem.
After filling the ground floor of the couple's Los Angeles mansion with roses and candles, Jason reportedly presented Britney with a flower-shaped diamond ring, according to The Sun newspaper.
An insider said: "She's on cloud nine."
The 'Toxic' hitmaker is also said to be keen to have another child, her first with Jason, but only wanted to do it once she had tied the knot again.
The source added: "Britney is desperate to settle down again and have more children. But she is very traditional and didn't want to start a family with Jason until they'd tie the knot."
Britney - whose first marriage to childhood friend Jason Alexander was annulled just 55 hours after they said, 'I do' in Las Vegas in 2004 - is now said to be planning a "low-key" ceremony in Hawaii once she has completed her upcoming world tour.