Campaigners have attacked a "war on women" being waged by religious organisations before an international summit on family planning to be held in London this week.

The conference, co-hosted by the Department for International Development (Dfid) and the Bill and Melinda Gates Foundation, plans to raise money and awareness to bring contraception to millions of women and girls in the developing world.

This weekend Melinda French Gates, the wife of the Microsoft founder and one of the world's richest women, tried to deflect controversy around the summit. In an interview to be broadcast on CNN on Sunday, she said giving women better access to contraception had become her lifetime's work.

Gates, who is a practising Catholic, has been targeted by religious groups, which have described her mission as a "blatant attack on morality" and an elitist effort at population control.

In response Gates said the lack of family planning available to 210 million women was "a crime". She added: "We made birth control and contraceptives way too political in the US. I think if people understood that 200 million women want this around the world they would start to say, 'OK that makes sense.'"

Her view is backed by Andrew Mitchell, the minister for international development, who told the Observer that no attention should be paid to "noises off" in setting a new agenda. "We have to focus on what we know there is widespread support for," he said. "It may be an edgy agenda, but we are very clear that what we are seeking to do is to reduce by half the number of poor women who want contraception but can't get it. We're trying to ensure that women have the opportunity to decide for themselves.

"We know it makes economic sense and, if we are successful, it would mean 100 million fewer unintended pregnancies, 200,000 lives saved, 50 million abortions averted," he said.

The London summit, which takes place on Wednesday, will kick off the Gates Foundation's official campaign – Gates wants to raise $4bn worldwide.

There is a strong consensus to suggest that with access to voluntary family planning, poverty declines, education rates rise, the health of women and children improves and the numbers of women who die in childbirth and children who die under the age of five falls.

Yet despite decades of campaigning, family planning became so politicised it fell off the agenda, becoming the "step-child of development", said Neil Datta, co-ordinator of the European Parliamentary Forum on Population and Development. He said three new anti-abortion groups had opened offices in Brussels in the past three years.

"The anti-choice websites are springing up too," he said. "There is without doubt a war against women and women's rights. But people who are against family planning also tend to be largely against any type of sexual relationship not within a heterosexual marriage which exists to produce a child."

Welcoming the summit, the sexual health charity Interact Worldwide said demand had to be created as well as supply. "For the summit to achieve any of its goals it must balance the supplies of commodities with resources for creating demand," said its chief executive, Marie Staunton, adding that a stigma was firmly attached to contraception.

Interact's research in the Indian state of West Bengal found that information, access to media and peer-to-peer discussions were vital to increasing contraception use. "It's a good start," said Staunton, "but for it to be transformative, we need to keep coming back to women's rights. These aren't abstract – they're tangible."

Anti-abortion campaigners in the US have seized on comments by a Harvard professor, Lant Pritchett, who attacked Gates for counting women who have not expressed a desire for contraception as needing it.

However, the experience of people on the ground is that, once women are offered education about contraception, it is welcomed. "There is a lot of ignorance," said Faustina Fynn-Nyame, country director of Marie Stopes International in Ghana. "Women think, for example, that an implant or coil will stab them. Once they see other women are fine, then they want to control their families. It empowers them to choose."

She said that she and her staff went home each night "completely exhausted… because we are fighting a battle on the ground here. It's a war against women, and not one we feel we are winning.

"This is an issue that we have ignored for far too long and every day women are dying and having children they struggle to feed and cannot send to school because they have too many. But they cannot choose how many they have. They deserve the right that women in the west take for granted," she said.

Campaigners are keen to make sure donors understand that family planning means shipments of pills as well as education. "Pregnancy-related deaths are the leading cause of mortality for 15- to-19-year-old girls, and those who give birth aged under 15 are five times more likely to die than women aged over 20. For this, and other reasons, family planning is important, but it's not as simple as ensuring contraception is available," said Anna Feuchtwang, chief executive of international development charity EveryChild.

"It has been reported that leaders at Rio+20 dismissed reproductive rights as a 'health issue', so this summit comes at a crucial time. Dfid and the Gates Foundation should be congratulated. We need to enable women and children to take charge of their lives, make positive decisions and keep families together."