Hong Kong's number two official Carrie Lam resigned on Thursday, local media said, in a move widely seen as a prelude to contesting a March election to become the Asian financial hub's next chief executive.

Lam has already submitted her resignation, Hong Kong's Cable Television reported, citing unnamed sources.

Her office gave no immediate comment.

Lam, 59, has served as the financial hub's Chief Secretary since 2012 overseeing the civil service. Known for her no-nonsense attitude as a tough and effective leader, she is widely known by the moniker the "fighter".

The bespectacled career civil servant, often wearing a traditional one-piece Chinese cheongsam dress, is seen as one of the frontrunners in the race to lead the former British colony governed under a "one country, two systems" principle since 1997, granting it a high degree of autonomy under Chinese rule.

Respected former Financial Secretary John Tsang, who resigned in mid-December, is also expected to compete for the top post but hasn't yet made a formal announcement.

Incumbent Beijing-backed leader Leung Chun-ying, surprised many in early December when he said he wouldn't seek a second five-year term after he leaves office in July, throwing the race wide open.

A committee of 1,200 people, most of them Beijing loyalists, will vet potential candidates before choosing the next leader for the city of 7.2 million people on March 26.