Karl Kulessa, from the UN Population Fund (UNFPA) said that Russia's population could fall from 142 million to as low as 100 million in 40 to 50 years
Russia lost between 400,000 and 650,000 people per year from 1992-2006, following the chaotic collapse of the Soviet Union and its social welfare net.
Life expectancy for men was 55 years in the 1990s, nearly 20 years less than in western Europe. Fewer children are also born in Russia.
An expanding economy needs a youthful workforce, especially in outlying areas such as the far east and Siberia, the UN report said.
"Efforts are being made to increase birth rates and reduce death rates. Both are showing some good results, but more can still be done," Kulessa said.
Vladimir Putin, Russia's president, has made a priority of fighting a falling population, unveiling policies to strengthen mothers' rights, promote the family and encourage healthy living.
Bureaucrats in Russia's Ulyanovsk province last September even urged residents to do their 'patriotic duty' and make love, with prizes for anyone producing a child on the country's constitution day, June 12.
The old Soviet Union encouraged motherhood by offering medals and financial incentives for women who produced large families to meet the demands of industrial expansion.
A booming energy and commodity prices have enriched Russia during Putin's eight years in power.
The country has saved billions of dollars to buffer economic shocks and improve infrastructure and people's quality of life.
But while both Putin and Dmitry Medvedev, Russia's president-elect, promote a healthy lifestyle by sticking to a strict regime of judo and swimming, sportsmen promote vodka in advertisements and single-child families are common.