A former head of Singapore's narcotics police force was cleared of corruption on Thursday, after a court rejected charges that he demanded oral sex from a contractor to help her win government deals.

Ng Boon Gay, former director of the Central Narcotics Bureau, was accused of giving confidential information to a technology supplier who testified for the prosecution, but her statements were found to be inconsistent and unreliable.

"I am satisfied that the prosecution has failed to hold a case against you beyond reasonable doubt," District Court Judge Siva Shanmugam told Ng after a trial which had captivated Singaporeans with its lurid details.

Supporters of Ng, 46, applauded after the verdict was announced. His wife was seen leaning her head on a relative's shoulder and wiping tears from her eyes.

Ng, one of the most senior public officials ever tried in Singapore for corruption, was accused of demanding oral sex from Cecilia Sue, 36, a computer systems sales executive, in exchange for helping her win contracts.

He admitted during the trial that he had sexual relations with Sue, but said that they had an extramarital affair that involved no corruption or coercion.

High-level corruption cases are rare in Singapore, which is known as a corporate and financial centre with an efficient and well-paid bureaucracy.

Ng was charged with four counts of corruption. He could have been jailed for up to five years and fined a maximum of S$100,000 (US$81,000) on each count.

The trial judge said that Ng's actions were "not corrupt” as he had been in an intimate relationship with Ms Sue the whole time.

"She did not impress me as someone who could be taken advantage of," the judge added.

State prosecutors are currently assessing the ruling.

"We will study the grounds of the decision and assess on whether to appeal," said a spokesperson for the Attorney-General's Chambers.

- AFP