By Julien Toyer MADRID (Reuters) - Spain's ruling People's Party is on course to win the most votes in a national election in December, a poll showed on Thursday, but the rise of the opposition Socialists and new party Ciudadanos has increased the possibility of a centre-left government. The closely-watched survey, released quarterly by the Sociological Research Centre (CIS), also showed that leftist Podemos, which ranked a close second earlier this year and raised the prospect of a Greek-style government in the euro zone's fourth-biggest economy, slipping to the fourth position. Coming third in the election is important because Spain's electoral law makes it difficult for more than three parties to obtain significant parliamentary representation.