South Korea's most powerful businessman said on Tuesday he would step down after 20 years at the head of the giant Samsung Group following his indictment last week for tax evasion and breach of trust.

The announcement by Lee Kun-hee came as a shock even in a society long used to its top businessmen being hauled into court.

"(I) deeply apologise for causing concern to the nation and will take full responsibility for that," an expressionless Lee said in a brief statement while dozens of senior group executives standing behind him watched on.

The group will dismantle its powerful strategic planning office, which critics say is an opaque organization able to wield influence across some 60 affiliates, including flagship company Samsung Electronics, a world leader in computer memory chips and flat display screens.

A Samsung spokesman told reporters the reshuffle was the start of a process that would give its companies more transparency, its management more autonomy and end a complicated cross-shareholding arrangement that gives Lee enormous power despite holding a tiny stake in group companies.

Lee's son, Lee Jae-yong, seen as being groomed to take over, will step down from his executive post and work abroad for the group in another, unspecified role.

Samsung group firms account for 20 percent of total market capitalization on the main board of the South Korean bourse The group has more than 250,000 employees and its annual revenues of $160 billion are around the size of Singapore's GDP.

A special prosecutor in January launched a probe into corruption allegations after a former top legal executive at the group said some of its top management hid money and kept a slush fund to bribe politicians, prosecutors and officials. The prosecutor indicted nine other top executives, but found no evidence to support the bribery allegation. If found guilty of tax evasion Lee could serve from five years to life in jail.