The chairman of Samsung Electronics, the world's biggest smartphone and chip maker, has been jailed for sabotaging union activities, prompting a rare apology on Wednesday from the firm.

Chairman Lee Sang-hoon and executive vice president Kang Kyung-hoon were both sentenced to 18 months for leading a wide-ranging operation to deter staff at Samsung's customer service unit from operating a union.

Samsung Electronics is the flagship subsidiary of the Samsung group, by far the biggest of the family-controlled conglomerates known as chaebols that have propelled South Korea's rise to the world's 11th-largest economy.

Along the way, the company has fought ferociously against union representation, until local authorities in Suwon, where it is headquartered, last month certified the National Samsung Electronics Union, which is affiliated with a powerful umbrella group.

Lee and Kang were found guilty by the Seoul Central District Court on Tuesday of violating labour union laws, with prosecutors saying they had ordered subordinates to cut union members' wages and to discover and exploit details of their personal lives such as pregnancies and debt, among other tactics.

Both men had been on bail during the trial but were immediately taken into custody.

At the time in 2013, Lee was Samsung Electronics' chief financial officer, and both men have been members of the now-disbanded Future Strategy Office, the firm's key decision-making body.

The company issued a joint statement on Wednesday with construction affiliate Samsung C&T - which has also had executives convicted of violating union laws - acknowledging their errors.

"We humbly accept that the companies' understanding and view towards labour unions in the past fell short of society's expectations," they said. They pledged to ensure no repetition, adding: "We will endeavour to build a forward-looking and productive labour-management relationship based on the respect for our employees."