The Hong Kong stock exchange was forced to suspend trading in stocks including HSBC Holdings after hackers broke into the exchange's website on Wednesday, preventing investors from accessing company announcements made during the midday break.

Our current assessment that this is a result of a malicious attack by outside hacking, the chief executive of Hong Kong Exchanges & Clearing (HKEx) Charles Li told reporters after the company announced interim results.

In a statement released earlier, HKEx said it had adopted a half-day (one trading session) suspension policy for issuers that announce price-sensitive information during the lunch hour.

Other systems at the exchange were not affected and trading in its securities and derivatives markets operated normally, the exchange said.

If the website remains unstable on Thursday, the exchange's bulletin board will be used for dissemination of information but the stocks will be not suspended, said Mark Dickens, head of listing at HKEx.

Li's statement comes after internet security firm McAfee said last week that it discovered a five-year long campaign of cyber attacks on the networks of governments, organizations and businesses.

Other targets have been the United Nations, the Association of Southeast Asian Nations, the International Olympic Committee; and an array of companies from defense contractors to high-tech enterprises.

HKEx had its own shares suspended after it reported a 19.5 percent rise in second-quarter profits. Other stocks which were halted for the day included Cathay Pacific and Dah Sing Financial which also reported interim results.

It was the first time for a suspension due to such a kind of technical problem and one involving so many companies, said Alfred Chan, chief dealer at Cheer Pearl Investment in Hong Kong.

HSBC, which comprises 15 percent of Hong Kong's benchmark, confirmed during the midday break the sale of its underperforming U.S. credit card business and retail services unit to Capital One Financial.

In all, stocks contributing to about 18 percent of the Hang Seng index's weight did not trade during the afternoon session.

The move to suspend trading hit turnover on the exchange in the afternoon session since HSBC and HKEx are amongst the most liquid names traded in Hong Kong.

Shares of HSBC last traded up 3.9 percent but could have pared gains into the afternoon session considering its London-listed shares were trading up just 0.1 percent as of 6:30 a.m. EDT.

(Additional reporting by Donny Kwok; Editing by Anshuman Daga)

HSBC Target