A bill lengthening Russian presidential terms passed its latest hurdle in parliament on Wednesday, amid speculation over the implications for the future of powerful Prime Minister Vladimir Putin.

The constitutional amendments won overwhelming approval in a second reading debate in Russia`s lower house of parliament, the State Duma, where 351 lawmakers voted for the change while only 57 voted against.

The extension of the presidential term from four years to six has been seen by some Kremlin critics as paving the way for Putin`s return to the presidency after he left the post in May.

Putin is due to give an eagerly-anticipated address to his United Russia party -- which dominates parliament -- at its party congress on Thursday. But it would be a major surprise if he revealed his future political intentions.

On Tuesday, Putin`s spokesman said the prime minister would continue his presidential tradition of holding a phone-in with the Russian people this year, further reinforcing the impression he is still the de facto leader.

The Duma will now need to vote again on the bill in a third reading before it goes before the upper house, the Federation Council. It must then be approved by two-thirds of Russia`s regional assemblies before becoming law.

The amendments would be the first change to Russia`s post-Soviet constitution since its adoption in 1993.

The Duma overwhelmingly approved a first reading of the bill last week, with only the Communist faction voting against the changes. It complained of an `authoritarian` trend in Russia.

Liberal parties critical of Putin are not represented in this parliament but Valery Borshchov, the co-chairman of the liberal Yabloko party, criticised the move at a protest outside the Duma on Wednesday.

`I think that it is dangerous to change the constitution and prolong the presidential mandate. It`s dangerous for democracy,` he said.

`I do not exclude there could be an election so that Putin returns` to the presidency, he added.

The Kremlin however has insisted the changes are simply aimed at `ensuring stable and progressive development of the country.`

Duma Speaker Boris Gryzlov, who is one of the leaders of United Russia, confirmed that Putin would be chairing Thursday`s party congress and Dmitry Medvedev, his successor as president, would also be in attendance.

`Questions will be discussed that affect the life of every citizen,` he was quoted as saying by the ITAR TASS news agency.

He said that Putin would also report on the country`s development to 2020 under what is called the `Putin Plan, a Worthy Future for a Great Country.`

Citing unnamed Kremlin sources, the Russian weekly Newsweek reported in its latest issue that Putin was planning to surrender the premiership and return to the presidency for two more six year terms.

It said that `firm and unpopular` measures were required at a time of mounting turmoil over the economy and it should be the job of a more technocratic prime minister to take them, rather than Putin.

Deputy Prime Minister Igor Shuvalov had already discussed with Putin the possibility of succeeding him, it said.

It was Medvedev himself who proposed the amendments to the constitution on November 5. They also envisage extending the term of the Duma to five years from the current four.

Another clause of the constitutional reform would require the government to answer questions in front of parliament regularly.

Medvedev insisted that the change would not herald Western-style parliamentary democracy for Russia: a `parliamentary republic` would be `like death` for Russia, he said.

Not all analysts are convinced that Putin is set to make a dramatic return to the Kremlin or that Medvedev would leave so easily.

`Medvedev is no longer a puppet of Putin but a real actor,` said analyst Yuri Korguniuk of the Indem foundation.