Spears: Waiting for a flight to the US
The star, who was on the last flight to leave America for Australia before the attacks took place, will return to be with her family.
"After much thought and deliberation it is with great regret that I have decided to cancel my planned European and Latin American promotion trips," she said in a statement.
She was due to appear in Cologne, Germany and Rio de Janeiro, Brazil after launching her third album in Australia.
She flew out to Sydney on the same flight as Keanu Reaves and US Open tennis champion Lleyton Hewitt, and were only told of the attacks in America when they landed.
"We are currently unable to get flights back to the United States, so we plan to stay in Australia until this is possible," Spears said.
"[I] trust you will understand my decision to return home and spend time with my family and loved ones."
Madonna will donate the proceedings from her three Los Angeles concerts to charity following the attacks.
Up to £70,000 could be raised at the shows, which took place on Thursday, Friday and Saturday.
The Backstreet Boys, who lost one of their stage crew on one of the hijacked flights, plus Sade, Earth, Wind & Fire and Lynyrd Skynyrd are among the other musicians who have pledged funds.
Name change
Major record companies Vivendi Universal, Bertelsmann and the EMI Group have said they will give a total of $8m (£5.4m) to relief efforts.
American electronic band I Am The World Trade Center have temporarily shortened their name after the attacks.
They will now be known as I Am The... and will donate part of the proceeds from their debut album Out of the Loop to the United Way's September 11th Fund.
And American rockers Jimmy Eat World have changed the name of their forthcoming single, Bleed American, to just Bleed in response to the events.