Special prosecutors said Tuesday they have delayed this week's questioning of President Lee Myung-bak's eldest brother over his role in the president's now-scrapped retirement home project by one day at his request.

Lee Sang-eun, the 79-year-old chairman of automotive seat maker DAS, is suspected of having played a role in the alleged irregularities by loaning some 600 million won ($542,000) in cash to his nephew and President Lee's only son, Si-hyung, for the land deal in which critics argue taxpayers' money was used.

The elder Lee will appear at the special counsel's office in Seocho-dong, southern Seoul, at 10:00 a.m. Thursday, said special prosecutor Lee Kwang-bum, who is leading the investigation.

He was supposed to meet with prosecutors on Wednesday, but requested the questioning be delayed by one day, citing health reasons, the prosecutor added.

The scandal involving the president and his family centers around allegations that the 34-year-old son and the presidential security service used public funds and violated real estate laws when they jointly bought a plot of land last year for the retirement home, which was to be built in Naegok-dong on the southern edge of Seoul.

The cost was allegedly not shared evenly, with the security service paying too high a price for the site for security facilities in what opposition parties have claimed was a scheme to allow the son to profit from buying the site at a below-market price at the taxpayers' expense.

Lee Sang-eun is expected to be quizzed over why he lent the 600 million won in cash to his nephew, who currently works at DAS, instead of simply transferring the money to his bank account, the counsel said. The elder Lee will also be grilled over where the money came from.

Lee returned home last week after leaving for China on Oct. 15, just a day before the team launched its investigation into the case. (Yonhap News)