Mr Putin says electoral changes are needed to strengthen statehood

The bill, which is expected to be adopted, would change the way parliamentary deputies are elected.

Members of the lower house, the Duma, would be chosen by proportional representation, with parties needing 7% of the vote to get seats.

Critics say it will be almost impossible for liberals to be elected.

Correspondents say such candidates attract only a small number of votes, but they are seen as important for the development of democracy in Russia.

Those supporting the move say it is based on the model used in the world's leading democracies.

Nationalists opposed

President Vladimir Putin says the changes, which will apply to the 2007 elections if approved, are needed to strengthen Russian statehood.

Why is this law needed? It stacks all the cards in favour of the ruling party Gennady Zyuganov Communist leader

The bill was passed by 335 votes to 96, with deputies from the Communist Party and their nationalist allies, Motherland, voting against.

It will now go to a third reading, before being considered by the upper house, the Federation Council. Mr Putin is then expected to sign it into law.

Under the current system, half of the 450 members come from parties which gain more than 5% of the vote, while the other half are elected in single-mandate constituencies under a first-past-the-post system.

In the December 2003 vote, only four parties crossed the 5% barrier, and the two main liberal parties, the Union of Right Forces and Yabloko, failed to do so.

DUMA PARTIES United Russia - main pro-Kremlin party, currently holds two-thirds of seats Communist Party - main opposition Liberal Democratic Party - Vladimir Zhirinovsky's ultra-nationalists, normally pro-Kremlin Motherland - Kremlin-created, but mostly in opposition

However, they were able to elect some individual representatives, and under the new system this would be denied them.

Small parties would also be banned from forming alliances to overcome the threshold.

Communist leader Gennady Zyuganov cried foul.

"Why is this law needed?" he said. "It stacks all the cards in favour of the ruling party."

United Russia deputy Vladimir Pligin said the bill was a step in the development of democracy.

"This law will set the right course toward building a mature civil society in this country," he said.

The Duma has already approved Mr Putin's controversial plan to end popular elections for provincial governors.