Former anti-narcotics chief Ng Boon Gay said he would never "throw a spanner in the works" in awarding contracts, as the keenly-watched corruption trial resumed on Tuesday.

Ng, 46, was replying to a question from his lawyer which referred to the prosecution's concept of "spanner influence" raised in the earlier stages of the case.

"I can't simply throw a spanner as and when I like," said Ng, the former head of the Central Narcotics Bureau.

He faces four charges of obtaining oral sex from former IT sales executive Cecilia Sue in exchange for furthering the business interests of her firms.

Ng's lawyer Tan Chee Meng also asked on Tuesday what he understood by "do you D-I-Y", a message Ms Sue sent to his phone last December. It meant whether he masturbated, said Ng, adding that she asked him this on multiple occasions.