President Vladimir Putin agreed on Tuesday to lead Russia's biggest political party, a move that will bolster his power after he steps down as Kremlin leader next month.

United Russia, which controls the lower house of parliament, created the post of party chairman specially to offer to Putin. Delegates at a party congress in Moscow immediately endorsed the appointment unanimously.

"I accept the invitation of the party. I am ready to take on myself the additional responsibility and head the party," Putin told them, to cheers.

"I promise to do everything to strengthen the influence and authority of the party and to use its potential for the good of the country's development."

Putin said he would serve as prime minister and take the party leadership after his protege, Dmitry Medvedev, is sworn in as president on May 7.

The move comes closer to solving Russia's biggest political riddle: How much influence will Putin have under President Medvedev?

Russian prime ministers usually play second fiddle to the president but analysts said the post of party chairman gives Putin extra clout as a future premier and indicates he will be a big political player for years to come.

"Basically, Medvedev will not be able to rule without Putin's agreement," said Alexei Pushkov, a professor of international relations and a leading journalist.

"Putin does not want to be what we call a technical prime minister in Russia, a person who can be removed in 30 seconds if the president wishes to do so; he wants to be a prime minister with his own power base."

CONTROL OF DUMA

Putin took the newly created job of party chairman for four years, giving him control of the Duma for the same period.

United Russia won 315 of the 450 seats in the State Duma (lower house of parliament) last December. Its two-thirds majority is enough to change the constitution or to start impeachment proceedings against the president.

Putin said the party - which rejects comparisons with the monolithic Soviet Communist Party - would have to reform itself to clear out "people who pursue only their own interests and personal advantage".

Medvedev told delegates that Putin's acceptance of the post of party chairman would help cooperation with the legislature, and that Putin had long been the informal leader of the party.

But the president-elect turned down an offer to join United Russia, saying it would be "premature" and that the head of state should not be directly associated with one party.

The two men enjoy a close personal relationship, forged over nearly two decades of working together, but diplomats have questioned whether they will be able to stop their respective teams from indulging in turf wars.

From the tsars through Communist times, Russia has long been accustomed to a single, all-powerful leader, and many analysts doubt whether a double-headed system of government can function effectively.

Opinions differ on whether Putin's plan is to shore up Medvedev - a 42-year-old lawyer who has never held elected office and is viewed with suspicion by Kremlin hardliners - or to control him.

Sergei Markov, a political analyst and United Russia member of parliament, told reporters at the congress:

"It (the appointment as party chairman) strengthens Putin's political weight as national leader. Dmitry Anatolyevich Medvedev is leader of the state and of the Russian Federation, but the political leader of the country remains Putin."