Liberal leader Grigory Yavlinsky is likely to be excluded from Russia\'s presidential race, election officials said Tuesday, a move that could undermine the legitimacy of Vladimir Putin\'s planned comeback. The Central Election Commission said it had examined 600,000 of the two million signatures the veteran leader of the Yabloko party submitted to qualify for the March 4 vote, and found that a quarter of them had problems. \"The number of signatures that have been deemed invalid and questionable gives the Central Election Commission grounds to deny registration\" to Yavlinsky, commission member Nikolai Konkin told reporters. A commission spokeswoman told AFP an official decision would be made by end of this week. The commission has in the past rejected signatures in support of anti-Kremlin candidates on various grounds. Analysts and the opposition said Yavlinsky\'s exclusion would further undermine the credibility of a vote already weakened by emerging claims of campaign violations and fraud-tainted December parliamentary polls that observers said were slanted in favour of Putin\'s ruling United Russia party. \"This significantly weakens a list of candidates that has already been made artificially narrow and turns the election into a joke,\" said political analyst Gleb Pavlovsky, who used to work with the Kremlin.  Yabloko branded the decision politically motivated, and said it expected further pressure from officials. Organisers of the next in a series of anti-Putin rallies adopted a resolution Tuesday demanding that Yavlinsky, 59, be allowed to run. Yavlinsky himself plans to speak at the demonstration on February 4. Prime Minister Putin is wrestling with the worst legitimacy crisis of his 12-year rule after he announced his plan to seek a third Kremlin term in a job swap with incumbent President Dmitry Medvedev. His approval ratings have taken such a beating that he may not poll 50 percent, forcing him to take part in a run-off for the first time, a humiliating prospect for a leader who once enjoyed sky-high ratings. Three other candidates -- Gennady Zyuganov of the Communist Party, Sergei Mironov of A Just Russia party and Vladimir Zhirinovsky of the Liberal Democratic Party -- have been registered. Independent candidate billionaire Mikhail Prokhorov, who critics say will run as a Kremlin-friendly candidate to soak up opposition to Putin, is also most likely to be registered. Both Prokhorov and Zyuganov came out in support of Yavlinsky, with the metals tycoon Prokhorov saying the system that requires two million signatures for registration was humiliating and should be abolished. Political analysts said the Kremlin wanted to disqualify Yavlinsky to ensure a decisive first-round victory for Putin, but questioned whether Yavlinsky\'s backers would actually switch to the current prome minister. Opposition leader Boris Nemtsov noted that the Yabloko party had many poll observers, but would not be allowed to monitor the vote if Yavlinsky is not running. \"A lower level of monitoring will allow authorities to cheat,\" he told AFP. The softly-spoken Yavlinsky made a surprise comeback to politics after Yabloko polled well among middle-class voters in Moscow and Saint Petersburg during the parliamentary elections. Yabloko however still failed to make it into parliament in a vote that the opposition and observers said was skewed in favour of Putin\'s party. A survey by state-controlled pollster VTsIOM earlier this month found that just one percent of voters would support Yavlinsky as president, compared to 52 percent for Putin.