An estimated 233 million women in their fertile years will lack access to modern contraception by 2015, up from 221 million in 2010, according to a study published on Tuesday.
Excluding China, developing countries will account for more than 80 percent of the unmet need, especially in sub-Saharan Africa, experts reported in The Lancet.
Use of contraception among women aged 15-49 has risen over the past two decades, it said.
But a demographic bulge means the total of women lacking access to fertility control will rise without funding to tackle the problem.
In 2010, 12.3 percent of women of fertile age did not have access to contraception, down from 15.4 percent in 1990, according to the study, funded by the UN Population Division and the National University of Singapore.
Under the Millennium Development Goals, UN members pledged to achieve "universal access to reproductive health" by 2015.
In its annual State of World Population report last November, the UN said family planning led to a boost in child health and education, helped women secure a place in the workforce and reduced dangerous back-street abortions.
If an additional 120 million women who wanted contraceptives could get them by 2020, an estimated three million fewer babies would die in their first year of life, it said.