Russian President Dmitry Medvedev has signed into law constitutional amendments extending presidential terms from four years to six.
A Kremlin spokeswoman confirmed that Medvedev had approved the amendments, after they were announced in early November and pushed through the national parliament and all 83 regional assemblies in less than two months.
Analysts have speculated the changes could be designed to pave the way for a return to the presidency by Prime Minister Vladimir Putin, who left the Kremlin in May but is often viewed as continuing to wield true power in Russia.
Last month the Vedomosti daily newspaper cited two Kremlin sources as saying that Medvedev would resign soon after the approval of the amendments and that Putin would then run in a presidential election in 2009.
Critics have criticised lawmakers' haste in approving the amendments, which are the first change to Russia's post-Soviet constitution since its adoption under former president Boris Yeltsin in 1993.
The amendments sailed easily through both houses of the national parliament, which is dominated by United Russia, a pro-Kremlin political party whose chairman is Putin.
Medvedev, who took over from Putin in May, has argued that the amendments will improve political stability.
He has said the changes will not apply to his current term, which is due to last until 2012, and would only affect the winner of the next election.
The amendments also extend parliamentary terms from the current four years to five and include a provision for the government to report to parliament on a regular basis.
The amendments still need to be published in an official government newspaper to come into force.
