The result could be labor shortages, an aging population and slower economic growth, the U.N. said.
Recent Kremlin efforts to reward women for having more babies have caused a surge in the birth rate, the report said, but won't make much difference in the long term.
It urged Russia to reduce its high mortality rate _ similar to that in parts of sub-Saharan Africa _ through reform of its public health system and by encouraging lifestyle changes _ especially a reduction in alcohol consumption.
The United Nations Development Program report, titled "Russia Facing Demographic Challenges," predicted that Russia will be forced to adapt to a smaller population and work force.
"Efforts to resist the unfavorable trends must be combined with efforts to adapt to what cannot be resisted," the report says.
Population levels in many developed countries have stagnated and are expected to fall by 2025, but Russia's population, currently around 142 million, has been in retreat since 1992. Russia's mortality rate is among the highest in the developed world, with average life expectancy for males at barely 60 years.
For reasons that are not fully understood, Russians suffer very high levels of cardiovascular disease. But most experts blame the country's overall high death rate on one factor, alcohol. It has been linked to everything from liver disease to Russia's high number of murders, suicides and fatal accidents.
According to a 2007 U.N. report, in 1950 what is now the Russian Federation had the world's fourth-largest population. By 2007, the report said, Russia ranked ninth globally, behind Bangladesh and Nigeria. By 2050, the U.N. estimates, Russia will rank 15th, with a population smaller than that of Vietnam.
Monday's report notes that population decline in general reduces a country's "strength and dynamism." The report adds that the effects of depopulation will be magnified in Russia because of its huge territory.
An influx of immigrants over the past 16 years has helped soften the impact of Russians dying young and having fewer children. But the report says that many of these immigrants were ethnic Russians returning to their homeland from other former Soviet states, and this is mostly over.
Meanwhile, many skilled Russians could be lured abroad in the coming decades, the report says, as labor shortages develop in Western Europe, where a shrinking pool of working-age people is expected to drive up wages for the highly educated.
To cope with this demographic crisis, the U.N. report recommends that the government overhaul the health system to provide more efficient care, while encouraging lifestyle changes to reduce the number of deaths related to alcohol consumption.
A study published in June in The Lancet medical journal found that drinking has caused more than half of deaths among Russians aged 15 to 54 since the 1991 Soviet collapse.
President Dmitry Medvedev began an effort to restrict beer sales in early September, citing the effect of alcohol on public health.
But the move is politically risky, especially in a country where alcohol in general and vodka in particular plays such an important cultural role. Former Soviet leader Mikhail Gorbachev's efforts to restrict alcohol sales were deeply unpopular.
Russia can help compensate for fewer births and high death rates, the report said, by encouraging immigration. It estimated that the country will need to attract about 15 million migrants by 2025 to fill vacant jobs.
But the report notes that migration can also lead to tensions. It says the country will have to make a major effort to assimilate migrants, who today face discrimination, exploitation and sometimes violene.
In order to ensure that Russia's shrinking work force does not slow economic development, the report said, efforts should be made to raise labor productivity.
In part, that means cutting employment in many faltering industries where Soviet-era labor practices linger and encouraging people to move to more productive jobs in modern high-tech industries.
The report also predicts that the number of students entering Russian institutions of higher education will fall by half in the coming decades, forcing universities and technical schools to compete for students in order to survive. The report predicts this trend could lower the quality of education and professional training, handicapping economic development efforts.