Former Prime Minister Han Myeong-sook, considered one of the leading contenders for Seoul mayor, said Tuesday that she has decided not to run in next month\'s by-election. The announcement came as a surprise as Han was widely believed to have all but officially decided to run while leading opinion polls among potential opposition candidates. Last week, she agreed with Park Won-soon, another leading candidate, to seek a unified opposition candidate. Han, who served as South Korea\'s first female prime minister in 2006-2007, narrowly lost to Oh Se-hoon of the ruling Grand National Party in the race for Seoul mayor in last year\'s local elections. Oh quit last month after failing to block an opposition-led free school lunch program in the city\'s first-ever referendum. Han, currently an advisor to the main opposition Democratic Party, gave no clear reason for her decision. In a statement read by Rep. Baek Won-woo, she said that she will devote herself to the opposition party winning in next year\'s parliamentary and presidential elections. \"What our people yearn badly for now is changes in our politics and a government change in 2012,\" Han said in the statement. \"I will use all my soul and might ... for a victory in the 2012 general elections and a change in the government.\" Han\'s decision effectively left Park, a liberal civic leader, as the only viable liberal candidate. Park has no party affiliations, but he agreed last week to form an alliance with the opposition party to field a unified candidate in the mayoral election. Also last week, star computer expert and professor Ahn Cheol-soo dropped his mayoral bid, saying he would instead back Park. Park is a civic leader who founded the People\'s Solidarity for Participatory Democracy, one of the country\'s largest civic organizations, in 1994 with the stated goal of reforming politics through grassroots activism.