South Korean President Park Geun-hye accepted the resignation on Monday of her prime minister, who had offered to step down over an allegation that he took illegal funds from a businessman who committed suicide in a scandal that threatens to weaken Park.
Park, whose ruling Saenuri Party faces a general election early next year for control of the 300-seat parliament, has seen her public support rating dip below 40 percent in recent weeks.
The main opposition party has called for the dismissal of Park's chief of staff and for an independent prosecutor to investigate claims made by the businessman, Sung Wan-jong, that he had given funds to Park's aides and key members of her party.
Prime Minister Lee Wan-koo has denied he received campaign funds from the businessman, but pressure mounted for him to step down after news reports showed he had known Sung well. That was despite his claim that he barely knew the construction company boss who was himself a former lawmaker and was close to politicians.
On Monday as he departed the post, Lee apologized for the controversy but did not directly mention the allegation against him or repeat his denial.
"There is a lot I want to say but I will leave behind a void with the belief that the truth will one day surely be told," he told government officials at a departure ceremony.
Sung, who was under investigation for fraud and bribery, committed suicide earlier this month.
Park's current and past chiefs of staff have denied accepting money from him. Park has not been personally implicated in the scandal.
Political contributions in excess of 100,000 won ($93) were made illegal under a campaign finance law passed in 2004.
A public opinion poll released on Monday put her support rating at 36.8 percent, sliding again after recovering slightly in March.
Park has faced fresh criticism for her handling of a ferry sinking that killed 304 people, with many victims' families unhappy that the government decided only last week to salvage the sunken Sewol, failing to announce a plan in time for the one-year anniversary of the disaster on April 16.
Park did not immediately name a new nominee for prime minister, a largely ceremonial post.