MOSCOW — Russian officials declared the Levada Center, the country’s only major independent pollster, a “foreign agent” on Monday, two weeks before nationwide parliamentary elections and days after a poll showed sliding support for the governing party, United Russia.

The decision, announced by the Justice Ministry, means the Levada Center will probably shut down its polling operations, which it has been conducting since the late 1980s.

“This manifests the increase in internal repressions carried out by the country’s leadership,” the center’s chief, Lev D. Gudkov, said in an interview broadcast by Dozhd, Russia’s only liberal independent news station. “If they won’t cancel this decision, it will mean that the Levada Center will have to stop working, because you cannot conduct polls with such a stigma put on you.”

A law signed by President Vladimir V. Putin in 2012 requires all nonprofit organizations that receive foreign funding and are engaged in loosely defined political activity to register and declare themselves as foreign agents, a term widely associated with spying in Russia. Rights activists have criticized the law as an instrument to marginalize independent civil society groups.