Vladimir Putin's ruling party on Tuesday faced accusations of breaking the law after its posters for Russia's legislative polls turned out to be almost identical to those of the election watchdog.

On December 4 Russia is holding key parliamentary elections which are seen as a dress rehearsal of March presidential polls in which Russian strongman and current Prime Minister Putin is expected to reclaim his old Kremlin job.

The Moscow City Election Commission overseeing the polls in the Russian capital has had scores of billboards installed around Moscow to encourage Russians to vote.

But those of Putin's ruling party United Russia which appeared this week on the Moscow streets are almost identical, in what opponents said was a blatant attempt to plant a subliminal message in the mind of voters.

The poll watchdog's posters decked in Russia's national colours of blue, red and white feature silhouettes of several generations of people including children, parents with a baby carriage and an elderly couple with a cane.

"For life, for people," read the red letters above a silhouette of the city.

The United Russia posters feature the very same silhouettes with the same city view in the background. The posters say in blue and red letters: "(We) develop/For life, for people". They call on Russians to vote for number 6, the ruling party's number on the ballot.

"How everything coincided," quipped independent broadsheet Vedomosti.

But the spokesman for the Moscow Election Commission Dmitry Reut denied the existence of violations on the part of the watchdog.

"We are not urging people to vote for United Russia," he told AFP.

Senior United Russia party member Valery Ryazansky, speaking on Echo of Moscow radio, admitted that the billboards were indeed similar but insisted that a "special commission" had approved the party's posters.

Grygory Melkonyants, deputy executive director at Golos Association, an independent non-profit organisation aiming to protect voters' rights, said the nearly-identical ruling party posters appeared after those of the poll watchdog and played into its hands.

"The principle of equality of all election participants is being violated," he told AFP.

Two of the ruling party's competitors -- Russia's Communist Party and the left-leaning A Just Russia party -- said they had filed complaints with the Central Election Commission, noting it was just one of many violations by the ruling party during the ongoing campaign.

"They have become so cheeky they think it is okay to fully merge with executive authorities," Alexander Ageyev, head of A Just Russia's Moscow branch, told AFP.

Senior Communist Party member Andrei Klychkov, speaking on Echo of Moscow radio, added: "Essentially this means that budget funds, including taxpayers' money, went into the United Russia's election campaign."

The controversy comes as approval ratings of the United Russia party, expected to win the most votes, show signs of crumbling just ahead of the polls.

A poll of 1,600 by the independent Levada Center released on Tuesday showed Putin's party lost nine percentage points in the span of one week and now stands at 51 percent, one of its lowest approval ratings in years.

Putin's ratings have also slipped, falling to 61 percent by November 1 from 66 percent on October 11, the poll conducted between October 28 and November 1 showed.

Ratings of outgoing President Dmitry Medvedev, who in September agreed to step aside after just one term in office, fell to 57 percent from 62 percent over the same period.