LONDON:  Contraceptives are "one of the greatest anti-poverty innovations the world has ever known", philanthropist Melinda Gates said on Tuesday, calling for family planning to be a global priority.

Access to birth control boosts economic productivity by allowing women to earn an income and leads to smaller families with more resources to spend on children's health and education, Gates said.

The Bill & Melinda Gates Foundation was co-hosting an international summit in London on family planning, where donors were expected to raise at least $2.5 billion to expand access to contraception.

"Contraceptives empower women. And empowered women ... well, they transform societies," Gates said, according to an advance copy of her speech.

The foundation will announce an additional $375 million for family planning over the next four years.

The summit comes at a critical time, with US President Donald Trump having said he will end funding to the UN Population Fund (UNFPA), the United Nations agency which deals with family planning, sexual and reproductive health.

The Trump administration also reinstated a policy blocking US funding to overseas groups that perform or provide information about abortion.

Gates said she was "deeply troubled" by the proposed cuts.

"If empowering women is more than just rhetoric for the president, he will prove it by funding family planning," she added.

Gates, who is married to Microsoft co-founder Bill Gates, said contraception allowed her to finish her education, pursue a career in technology and plan when she had her children.

"My family, my career, and my life are the direct result of having access to contraceptives," she said.

But many women around the world get pregnant "too young, too old and too often", she said.

"In Malawi, everyone I met told me they knew a woman who had died in pregnancy," she said.

"In India, I sat in a circle of women and asked if anyone had lost a child. Every single woman raised her hand."

Some 214 million women and girls in developing countries cannot get access to contraceptives, experts say.

Meeting that need could help avert 67 million unintended pregnancies every year, thereby preventing the deaths of 76,000 women from pregnancy and childbirth related complications, they say.

Universal access to reproductive health services would lead to economic benefits of more than $430 billion a year as well, they say.

"Family planning has the power to change a whole country's trajectory and help lift people out of grinding poverty," Priti Patel, Britain's minister for international development, said in a statement.

Gates said contraceptives were "one of the smartest investments countries can make".

"At the individual level, contraceptives make lives better. In the aggregate, they transform economies."

The summit, held on World Population Day, will focus on ramping up access to contraceptives for adolescents, boosting family planning services in humanitarian crises and improving the supply and range of contraceptives available.

The conference comes five years after activists, philanthropists and leaders gathered at an earlier summit and set a goal to empower 120 million additional women and girls in the 69 poorest countries to use modern contraception by 2020.

The number has only grown by 30 million since then, meaning the target will be missed unless efforts are accelerated, organisers said.