The leader of South Korea's main opposition party, which polls say is headed for a big victory in an April election, said on Wednesday it would repeal a free trade deal with the United States if it wins control of parliament, unless the pact is renegotiated.

Parliament, currently controlled by the ruling conservatives, approved the deal amid rowdy scenes of opposition lawmakers protesting in November, after it was signed in 2007 by the then-government of left-leaning President Roh Moo-hyun. It is scheduled to go into effect in coming weeks.

The deal, which economists say could boost the $67 billion in annual trade between the two countries by as much as a quarter, was a highly emotive legislative issue that left the two sides in deep division, paralyzing parliament.

"The FTA pursued by the (President Lee Myung-bak) government has no regards for our national interest and it is our position that it cannot be left to go into effect," opposition Democratic United Party leader Han Myeong-sook told a party meeting.

"It must be renegotiated before it takes effect to fix the poisonous clauses. Otherwise, we will repeal it in the new parliament session," she said.

Though it was the opposition which initiated the FTA deal when it was in power, its legislators argue that subsequent changes to allow U.S. carmakers a major inroad into the market and a dispute settlement mechanism will strip Seoul of any ability to defend its interests.

The Democratic United Party released a text of what it said was an open letter to U.S. President Barack Obama asking for a renegotiation of the deal, saying "scores of South Koreans have taken to the streets to demand the repeal of the ratification."

"If our sincere request is overlooked by the U.s. government, and, as expected, if our party becomes the majority party after the next election, we will be taking all available measures to abolish the South Korea-U.S. FTA."

Public response to the deal has been generally positive, with expectations for growth in the country's auto and electronics sectors, but the small but politically powerful farm lobby and left-leaning parties argue Lee compromised the national interest in favour of big corporations.

The deal is the biggest U.S. trade pact since the North America Free Trade Agreement went into force in 1994. (Reporting by Jack Kim; Editing by Jonathan Hopfner)