S Korean leader's poll campaign probed South Korean prosecutors are investigating whether President Lee Myung-bak's 2007 election campaign received money from bribes allegedly paid to his brother by bank executives. Lee Sang-deuk, the president's older brother and one of his closest advisers, was arrested on Wednesday morning in connection with a corruption investigation. He is accused of receiving more than $500,000 from the chief executives of two struggling Korean savings banks between 2007 and 2011. Choi Woon-shik, the lead prosecutor in charge of the investigation, told the Financial Times that his team was trying to establish how the older Mr Lee used the money. "We will look into where the money was spent, either for personal purposes or for [Lee Myung-bak's] presidential election campaign," he said. A spokesperson for Lee Sang-deuk, who denied the charges in a preliminary court hearing on Tuesday, declined to comment, as did the office of the president. The investigation is being pursued as Lee Myung-bak enters the final months of a five-year term in which his popularity has plunged amid popular discontent with his conservative policies. It will heighten the widespread suspicion that Mr Lee's presidency has been marked by an excessive friendliness towards big business. The prosecutors claim that the chief executives of the Solomon and Mirae savings banks bribed Lee Sang-deuk in the hope that his political influence could stave off regulatory investigations into the banks. Both groups were among several South Korean savings banks suspended in the past year after it emerged that they had breached capital adequacy requirements. Analysts say Lee Myung-bak's reputation has also been damaged by allegations of corruption against close political allies. Ahn Byung-jin, a professor at Kyunghee Cyber University, said this case could have particularly serious implications. "Lee Sang-deuk is not just a key adviser to the president - he's kind of a godfather figure," Mr Ahn said. Lee Myung-bak depends on him very much. The arrest of the president's brother comes as South Korea fights to boost its reputation for transparency. Roh Moo-hyun, president between 2003 and 2008, committed suicide three years ago amid an investigation into the alleged bribery of members of his family. It is also a blow to the ruling New Frontier Party, for which the older Mr Lee was a longstanding legislator, ahead of a presidential election in December. However, Mr Ahn said the case was unlikely to seriously damage the campaign of Park Geun-hye, tipped to become New Frontier's candidate for president. "She has managed to successfully distance herself," he said.