Kwok, a former Independent Commission Against Corruption deputy commissioner, backed chief executive Leung Chun-ying during his election campaign.

His comments come as ICAC investigations into four of Hong Kong's most senior officials - past and present - continue and as corruption trials await two other former senior officials.

"When the ICAC tell them [the complainants] they will start to investigate, they tell the press about it in a high-profile way as if the person under investigation had been convicted," said Kwok, who retired from the ICAC in 2002 after 33 years of service.

He said the anti-graft agency pursued cases even if evidence was weak and in many cases the opening of an investigation was a legal requirement and largely procedural.

Critics say Kwok's comments could detract from the ICAC's legal obligation to take seriously all complaints - especially against the rich and powerful - wherever they come from.

Separate investigations into corruption allegations, some more serious than others, are under way into Leung, his predecessor Donald Tsang Yam-kuen and Henry Tang Ying-yen, who stood against Leung in last year's race for the top job.

Former government No2 Rafael Hui Si-yan is set to appear in the city's biggest corruption trial later this year alongside billionaire property tycoon brothers Thomas Kwok Ping-kwong, 61, and Raymond Kwok Ping-luen, 59 - the co-chairmen of Sun Hung Kai Properties.

Last July, then-development minister Mak Chai-kwong and assistant highways director Tsang Kin-man were arrested with their wives. They are suspected of leasing their flats to each other in the 1980s, allegedly cheating the government of housing allowances.

The controversy forced Mak to step down just days after taking office. Both men were charged in October and await trial.

And executive councillor Franklin Lam Fan-keung, a major real estate investor, is being investigated over the sale of two flats weeks before the government introduced new curbs on the property market in October.

Section 30 of the Prevention of Bribery Ordinance makes it an offence to disclose the identity of a person under investigation. But the legal source pointed out that trying to take action under that provision could be a minefield.

"There's no doubt the process is becoming politicised but can you imagine the outcry if action was taken ... against someone who had told the media about a complaint they had made against a politically powerful figure? The media would have a field day," the source said. "Perhaps the ICAC needs to close things down quicker."

But League of Social Democrats vice-chairman Avery Ng Man-yuen, who made the complaints against Tsang, Lam and Leung, said all the complaints had grounds. "All the cases have reasonable doubts ... that the officials had done something unlawful," he said.

Barrister and former ICAC investigator Stephen Char Sik-ngor said the commission had always encouraged people to make complaints and the public would be able to judge if they were legitimate or not.

James To Kung-sun, a Democratic Party lawmaker and vice-chairman of the Legislative Council's security panel, said the fact there were so many high-profile cases indicated corruption had become more serious.

"Some cases like the Rafael Hui case have been taken to court. We cannot say the situation is not serious," he said.

Despite suffering staff shortages, the ICAC is investigating more high-profile cases at once than at any time in its near 40-year history.

Last year, former ICAC commissioner Timothy Tong Hin-ming had his term extended for the second time - until June 30. He was succeeded by former director of immigration Simon Peh Yun-lu. Former head of operations Daniel Li Ming-chak also had his term extended twice.

It was widely expected that director of operations Rebecca Li Bo-lan would succeed Daniel Li. But instead the ICAC drafted in retired director of operations Ryan Wong Sai-chiu head of operations.

Legco recently approved funding for the ICAC to hire an extra assistant director of operations to help handle the huge Sun Hung Kai investigation.

In The ICAC's Sights

• Chief Executive Leung Chun-ying faces a fresh ICAC graft investigation over the chief executive election last year, after his former ally Lew Mon-hung turned on him in an interview last month with iSun Affairs magazine. Lew said Leung lied over his handling of illegal structures at his Peak homes and considered pan-democrats "enemies". It is the second time Leung has been under investigation by the ICAC since he assumed office. In December, it dropped a probe of a complaint alleging he gave false statements over at least six illegal structures at his homes.
   
• Former chief executive Donald Tsang Yam-kuen is under investigation for accepting favours from tycoon friends. He is accused of accepting advantages in connection with overseas trips and a bargain retirement penthouse deal over the border.
   
• Former chief secretary Henry Tang Ying-yen: Tang's bid to become Hong Kong's chief executive was torpedoed after an illegally constructed 2,250 sq ft basement was discovered at his home in February last year. The construction of the basement is now under investigation by the Buildings Department.
   
• Former chief secretary Rafael Hui Si-yan: Hui was arrested at his home in March last year. Also held were brothers Thomas Kwok Ping-kwong, and Raymond Kwok Ping-luen, who control Hong Kong's biggest property developer, Sun Hung Kai. The probe centres on Hui's alleged relationship with Sun Hung Kai over several years when he was in and out of government. The three have been charged with bribery and misconduct in public office.
   
• Mak Chai-kwong was forced to resign as development minister after 12 days amid an investigation into alleged abuse of housing allowances when he was a civil servant. He pleaded not guilty to fraud last month.