Rich countries have pledged $2.6bn over the next eight years at a family planning summit in London, in what was described as a breakthrough for the world's poorest women and girls. The money, coupled with commitments from developing countries, is expected to provide access to family planning for 120 million women in the global south.

"This will be a breakthrough that will transform lives," said the UK international development secretary, Andrew Mitchell. "The commitments made at the summit today will support the rights of women to determine freely, and for themselves, whether, when and how many children they have," said Mitchell at a conference hosted by the Gates Foundation and the Department for International Development (DfID), designed to put what has been a politically loaded issue back on the global development agenda.

More than 20 developing countries made commitments to boost spending on family planning and to strengthen women's rights to ease their access to contraception.

The summit's organisers say commitments made at the summit will result in 200,000 fewer women dying in pregnancy and childbirth, more than 110m fewer unintended pregnancies, over 50m fewer abortions and nearly 3 million fewer babies dying in their first year of life.

The conference sought to reverse two decades of neglect on family planning, especially during the Bush years. The event brought together several African leaders – including Rwanda's President Paul Kagame and Tanzania's President Jakaya Kikwete – NGOs and the private sector, and featured an unannounced drop-in by David Cameron.

The prime minister received a warm welcome for his strong advocacy of women's rights and of the UK's aid programme. "Women should be able to decide freely and for themselves whether, when and how many children they have," he said. "It is absolutely fundamental to any hope to tackling poverty in our world."

Besides pledges from donor countries, the conference heard commitments to expand family planning programmes from a parade of health ministers from developing countries. Malawi said it would raise the minimum marriage age to 18, India said it planned to have universal access to family planning by 2020, and Senegal said it would invest in a mass-communication campaign involving religious and political leaders.

"The Catholic church is with us as family planning is consistent within the context of marriage," said Senegal's health minister, Dr Awa Marie Coll-Seck. "As for Muslim religious leaders, we have some on the family planning co-ordinating committee. If the religious leaders are with us, we can really make headway."

The EU development commissioner, Andris Piebalgs, said he was heartened by the commitment shown by developing countries at the summit. "The first success is that developing countries have been very active," he said. "It's not the case that donors have driven the event."

The EU on Tuesday pledged €23m ($28m) ahead of the summit.

"Helping to provide family planning services is one of the best investments that a country can make in its future," Piebalgs said. "In today's world, all women must have the ability to choose the size of their families. It is about promoting gender equality and women's rights; but it is also about protecting maternal and child health."

The US has not pledged money on the grounds that it has already committed to spending $640m this year. For Rajiv Shah, the head of the USAid development agency, the summit was important to encourage "new donors to enter this space". Shah insisted that the US, where family planning can be conflated with abortion by the religious right, had bipartisan backing "because we promote family planning on a voluntary basis. It reduces unwanted pregnancies and abortions."

NGOs welcomed the focus on cultural attitudes as well as on increasing resources for family planning. "People have come here not just with specific and tangible pledges but have been willing to tackle thornier issues of culture. You have to tackle both condoms and culture," said Justin Forsyth, chief executive of Save the Children. "Girls in remote areas have never heard of contraception, even if they have, unless there are changes of attitude, things won't change. DfID and the Gates Foundation emphasise both sides of the coin, but the question of culture has to be addressed by developing countries themselves. I'm encouraged that those governments are also emphasising those issues. It feels like we are getting a common approach from donors, NGOs, developing countries and the UN."

Forsyth said another benefit of the summit would be its galvanising effect on family planning in much the same way that the recent London summit on vaccines attracted large donations. The summit, he added, made the wider case that aid works and can make a difference at a time of austerity.

The aim of the London summit on family planning is to raise $4bn to expand access to contraception for 120 million women in the global south by 2020. According to the UN, about 220 million women in the south who do not want to get pregnant cannot get reliable access to contraception.

The UK has committed £516m ($801m) over eight years to achieving the summit goal of enabling an additional 120 million to have access to modern methods of family planning by 2020.

Just as important as the pledges of money was the sentiment that women's and reproductive rights lay at the core of family planning. "Commitments are wonderful, but for them to work women have to be central in decision making," said Theo Sowa, interim chief executive of African Women's Development Fund.

• This article was amended 12 July to reflect the fact that USAid is spending $640m this year, not over eight years