Campaigning and advocacy organisation ONE International has written an open letter to world leaders, urging them to work harder to ensure that young girls in some of the world's poorest countries have access to education.

81 of Hollywood's biggest stars, including Lady Gaga, Charlize Theron, Natalie Portman, Ryan Reynolds, Blake Lively, and U2 frontman Bono, who co-founded ONE in 2004, have signed the letter, which will be delivered to world authorities on March 8, International Women's Day.