Sir Elton John and Jessie J are among more than 70 celebrities who have urged world leaders to tackle poverty by addressing gender inequality.
The pop stars joined U2 singer Bono, boxer Muhammad Ali and actress Charlize Theron in adding their voices to the call by the ONE Campaign ahead of International Women's Day.
In an open letter to government, the campaigners said "poverty is sexist" and some 155 countries have laws which discriminate against women.
The letter said: "Nowhere on earth do women have as many opportunities as men. Nowhere.
"While the debate around this truth rages everywhere, girls and women living in extreme poverty - those often hardest hit by the injustice of gender inequality - have been left out of the conversation.
"This must change. The fight for gender equity is global."
The ONE Campaign, co-founded by Bono, said Niger in west Africa had come out top in the rankings of the "toughest" countries in which to be born a girl.
Its analysis, in a report called Poverty Is Sexist, said girls in the country have fewer education and economic opportunities, including being unable to open a bank account.
The open letter adds that across the world some 63 million girls are denied access to education and half a billion women cannot read.
It calls for more funding to help girls and women fight HIV and malnutrition ahead of two summits - Nutrition For Growth and Global Fund - this year.
The group said figures suggesting girls account for three-quarters of new HIV infections and 40% of women in Africa suffer anaemia were an "outrage".
Sir Elton, founder of The Elton John Aids Foundation, said the numbers of young girls affected was "heartbreaking".
"Stopping the spread of HIV will mean doing more to protect girls and young women. The Global Fund is doing powerful work helping treat and prevent Aids and deserves expanded support from world leaders," he said.
"We have the chance to stop HIV/Aids in our lifetime, and we need to raise our voices now to make sure it happens."
Diane Sheard, UK director of the ONE Campaign, said: "The British Government have been at the forefront of efforts to put girls and women at the heart of international development and they should seize the opportunities of this year's major summits to cement this record.
"As hosts of the first Nutrition For Growth summit and major historical contributors to the Global Fund, now is not the time for the UK to take a back seat."
International Women's Day will take place on Tuesday and events to mark it include a Women Of The World festival at London's Southbank Centre.