Professor Song Du-yul returned to the south last year after more than 30 years of exile in Germany.

The case has divided the country at a time when it is trying to improve relations with the Communist north.

For decades Song was seen as a champion of democracy in South Korea - an outspoken critic of the country's former military rulers, even in exile.

When he returned to Seoul last year, he received a hero's welcome from liberal politicians.

But state prosecutors saw a more sinister purpose in his political activities.

Frequent visits

Song was arrested on charges of working for North Korea, a violation of the anti-Communist national security law.

The central district court has concurred that he was a secret member of the North Korean politburo and has sentenced him to seven years in prison.

Song was found to have visited the north on 18 occasions and to have sent letters promising his allegiance to the government here.

The case has been highly controversial at a time when the South Korean government is pursuing a policy of reconciliation with the north.

Senior members of the government are reported to have opposed legal action against him.

There was sympathy on the left for his stated attempt to bridge the gap between the rival states.

Analysts say the case highlights the dilemma facing liberals in South Korea; they have long focused their attacks on the right-wing establishment in the south, while taking a more tolerant view of repression and totalitarian control in the northern half of the peninsula.