The former head of the Singapore police's drug enforcement unit was charged Tuesday with corruption for soliciting sexual favours in exchange for help with contracts, court documents showed.

Ng Boon Gay, 46, former director of the Central Narcotics Bureau, appeared in a district court accused of "corruptly" obtaining "sexual gratification... from one Cecilia Sue Siew Nang" on four occasions, a charge sheet read.

He is the second high-ranking civil servant to face prosecution on sex-for-business charges in a week after similar accusations were filed against former civil defence chief Peter Lim last Wednesday.

According to the charges against Ng, he had oral sex with Sue four times, twice when she was a sales manager with Hitachi Data Systems Pte Ltd and twice when she worked for Oracle Corporation Singapore Pte Ltd.

Ng's liaisons served "as an inducement... to further the business interest" of the two information technology product vendors, the document read, in violation of Singapore's tough anti-corruption law.

It did not give specific details and Ng's lawyers told AFP he would contest the charges.

"Personal indiscretions aside, Boon Gay firmly believes he is not a corrupt officer," they said in an emailed reply to AFP queries.

They said they would seek further details from the deputy public prosecutor on how Ng had allegedly assisted in helping the two companies' interests, describing the charges as "very general in nature".

In a separate media statement, Ng's wife said she did not doubt her husband's professional integrity and would continue to stand by him.

If found guilty, Ng faces a maximum five-year jail term and a Sg$100,000 ($78,000) fine on each charge.

Large-scale graft cases are rare in Singapore, a thriving global financial centre and business hub which enjoys a reputation for being the least corrupt country in Asia.

It pays its civil servants some of the highest government salaries in the world in what it says is a deterrent to corruption.

Ng was placed on leave in December last year when the investigations started, the interior ministry said in a statement in January, and he was replaced in February.