MOSCOW (Reuters) - A proposal to extend the Russian president's term by two years moved closer to becoming law on Wednesday when the lower house of parliament approved it on its penultimate reading.

Kremlin critics say the extension of the presidential term from four to six years is part of a scheme to return powerful former president Vladimir Putin to his old job, but officials deny any such plan.

In the second of three readings, the State Duma lower house of parliament voted by 351 to 51 to approve amendments to the constitution extending the presidential term to six years and the Duma's own term to five years from four.

Communist lawmakers were the only group that voted against the amendments. The change in the Kremlin term was proposed by President Dmitry Medvedev, but it will not apply to the term he is serving now.

Some analysts say the constitutional amendments could be used as a legal justification for calling an early presidential election in which Putin, who now serves as a powerful prime minister, could run.

Putin's spokesman has said there is no plan for an early presidential election.

(Reporting by Aydar Buribaev; Writing by Christian Lowe; Editing by Louise Ireland)