"I have been discussing this with her for a few months," Cheung said yesterday.

"In fact, I have urged her to stay many times. In the end she decided to give priority to her family, so don't associate this with other matters. There are no other factors."

When asked whether Mok's resignation was mounting evidence of a governance crisis amid the departure of several other senior government officers, he said: "It is not related to governance. It is a personal decision."

Cheung said her stepping down would not affect civil servants' morale.

He said he understood that she had to take care of her two children, and the matter had been troubling her for a few months. He had tried working it out with her by adjusting her work arrangements, but in the end she had decided to leave.

Chief executive Leung Chun-ying said: "Time will prove that her reason for leaving is to be a full-time mother."

Mok's last day on the job will be August 24.

Ronald Chan Ngok-pang, a political assistant with the Constitutional and Mainland Affairs Bureau, said in a television interview: "There are certain challenges in engaging in political tasks in this time, but those willing to join the accountability team have the heart to serve society. I believe our colleagues will be dutiful in their posts."

The most recent resignations from Leung's team have included executive councillor Franklin Lam Fan-keung and Henry Ho Kin-chung, who was political assistant to development minister Paul Chan Mo-po.

Lam's departure on August 1 was prompted by criticism last year that he had made use of insider information to sell two flats ahead of new stamp duties, though no charges were found warranted.

Ho quit a day later, admitting he had failed to declare that his family owned land in Kwu Tung earmarked for a new town.