Spying cases involving Singaporeans are rare, but Dickson Yeo's is not the first to have been uncovered. In 1980, a cypher officer working at the Singapore Embassy in Moscow was jailed for 10 years after he was caught passing secret information to a Soviet intelligence operative.

Alan Wee Kheng Soon, then 30, was ensnared by a female agent, or "swallow", who seduced and blackmailed him into handing over information on how to code and decode messages being sent between Singapore and its embassy in Moscow.

The woman, who told Wee that her name was Luba Lubov Maluba, telephoned him at the Moscow apartment where he was staying with his wife and their two-year-old daughter.

She claimed to be a friend of the apartment's previous tenant, also a Singaporean, and the pair struck up a conversation.

They later met at a restaurant nearby and ended up spending the night together.

The Soviet agent eventually revealed that she was after the cypher code used to encrypt messages being sent to and from the Singapore Embassy. She threatened to expose Wee's affair to his wife and also threatened to harm his family if he did not comply.

Wee did not inform his wife or the ambassador about the incident.

Instead, he gave in to the demands and started handing decrypted messages to the woman while continuing to have sex with her on occasion. He eventually gave her the codes she wanted as well.

Wee was finally caught by Finnish police in Helsinki for smuggling. He attempted to stay in Moscow to avoid facing the music back in Singapore, but the Soviet woman declined to help him.

Senior District Judge Michael Khoo, who oversaw Wee's trial, said when passing judgment that his acts "undermine and greatly damage the security and interests of Singapore".

Wee pleaded guilty and received the maximum five-year sentence for each of the two charges of divulging secret information.

The judge ordered that the sentences be served consecutively due to the "utmost gravity" of the offences.