HONG KONG - Hong Kong authorities said Saturday it is investigating the wife of the man seen as China's preferred choice to become the city's next leader over an illegal "underground palace" at her home.

In the biggest blow to his campaign so far, Henry Tang came under pressure to quit the race for the chief executive after an unauthorised basement den covering 2,250 square feet (209 square metres) was found at the upmarket house.

Local media dubbed the basement an "underground palace", which reportedly featured a wine cellar, entertainment suite and a workout room. Many of Hong Kong's seven million people live in spaces a quarter of that size.

The city's buildings department has given the Tangs a week to remove the illegal structures, but it announced late Friday that it would probe Tang's wife, Lisa Kuo, and others under offences of "knowingly" contravening the law, which requires that approval is obtained before building work starts.

The house is registered under Kuo's name. The department did not name Tang specifically, who co-owned the house with Kuo until 2010. Those found guilty under the offence face up to two years in prison and a fine.

"We have asked the relevant parties, the architect, the contractor and the owner to assist in investigation," department director Au Choi-kai said Saturday, adding that no timeframe had been set for the probe to conclude.

The 59-year-old Tang tendered a public apology Thursday after saying the illegal structure was his wife's idea, a scandal which angered Hong Kongers with some saying his credibility was "bankrupt".

The avid wine collector, who is the former number two in the city's government, however refused to quit the March 25 chief executive polls, saying he should be judged by his business-friendly manifesto.

His comments -- taking responsibility while blaming his wife -- left many wondering if the man reported to be Beijing's favourite for the chief executive job has what it takes to lead the southern banking and trade centre.

Tang's campaign was already beset by his frequent verbal gaffes and opinion polls putting him well behind his main rival, Leung Chun-ying.

The "underground palace" scandal came as Tang's popularity rating plunged further by five percentage points to 21.3 percent in the latest opinion poll -- commissioned by the English daily South China Morning Post -- published Saturday.

Leung was favoured by 49 percent of the 1,007 respondents polled.

Tang has the support of the city's powerful business tycoons but his campaign got off to a shaky start late last year when he publicly admitted to cheating on Kuo, his wife of 27 years.

A 1,200-member Electoral Committee packed with pro-Beijing notables will choose the next chief executive, replacing incumbent Donald Tsang whose mandate is ending. AFP