The UK is pledging new money for contraception to help prevent the deaths of thousands of women in poor countries as a result of unwanted, unsafe pregnancies and backstreet abortions.

Stephen O'Brien, international development minister, who made the announcement at the world's biggest family planning conference in Dakar, Senegal, said the money was needed to increase women's access to family planning, which is "absolutely four-square within our over-arching policy and strategic imperative to put girls and women front and centre of all our development policies".

"You get it right for girls and women – you get it right for development," he said.

The money will prevent more than 2 million unintended pregnancies and avert nearly 220,000 unsafe abortions, says the UK Department for International Development (DfID), saving the lives of an estimated 3,700 women.

The UK has been one of the largest donors and supporters of family planning in developing countries and increased its funding when the Bush administration in the US bowed to the anti-abortion lobby and stopped payments to any overseas organisation involved in helping or even counselling women who were seeking a termination.

O'Brien said the UK position was very clear. Women and girls should be able to choose whether they want to try to start or enlarge their family.

"In the context of giving girls and women the choice, if the risk is that they would have an unsafe abortion, it is our intent that we should avert that unsafety. We don't duck that. It is averting nearly a quarter of a million unsafe abortions. [If safe abortion services were not available] they would happen anyway."

The new funds will also help prevent serious log-jams in the supply of hormonal contraceptives and condoms. Britain's £35m ($54.7m), on top of a commitment made in 2007 to the UN Population Fund (UNFPA) for £100m ($156m) over five years, will enable a rapid-response-style unit to procure and deliver supplies to countries facing stock-outs while governments work on better long-term arrangements.

The availability of condoms, hormonal implants and long-lasting injections is very low in much of sub-Saharan Africa. In the UK, 82% of women have access to modern contraceptive methods, but in Mali it is only 8% and in Chad, only 1.7%.

The £35m will be split between two UNFPA programmes that are working to boost supplies in places where they are most needed. The bulk of the money will go to the Global Programme to Enhance Reproductive Health Commodity Security, which is working in 12 key countries – two-thirds of them in sub-Saharan Africa. They include Niger, where two-thirds of the 15 million people are younger than 25, and 60% of girls are married before the age of 15, and Sierra Leone, which has the highest known rate of maternal deaths and a high fertility rate.

Some of the many deaths in childbirth in the developing world are the result of women having too many babies, beginning at too young an age – often while they are still at school and forcing them to drop out of education. Others are caused by women attempting to abort a pregnancy they did not want, often because they cannot afford another child.

A woman dies in pregnancy or childbirth every two minutes and 99% of the deaths are in the developing world. Every year, it is estimated that

215 million women have a pregnancy they did not want.

DfID says family planning is very good value for money. A year's support for one woman, including the cost of contraceptive supplies, costs less than £1 (around $1.50).