HONG KONG (REUTERS) - Thomas and Raymond Kwok, the billionaire co-chairmen of Sun Hung Kai Properties Ltd, had additional charges filed against them on Thursday in a bribery investigation surrounding Asia's largest developer, the company said on Thursday.

The Sun Hung Kai probe, Hong Kong's biggest corruption case since its anti-graft agency was formed nearly 40 years ago, involves one of Asia's most powerful families and the world's second-largest property company with a market capitalisation of US$33 billion (S$41.77 billion).

Thomas Kwok received an additional charge under the anti-graft law, while two additional charges for conspiracy to commit misconduct in a public office were laid against Raymond Kwok, the company said in a statement on the Hong Kong stock exchange.

The case will not affect its business and operations, the company said.

The Kwok brothers and Rafael Hui, the former head of Hong Kong's civil service, were charged in August 2012 for offences linked to bribery and misconduct in public office.

Two others have also been charged in the case including Thomas Chan, the board member in charge of land purchases at Sun Hung Kai Properties, and Francis Kwan, a former banker.

Shares in the company fell 0.2 per cent on Thursday morning, in line with a 0.3 per cent fall in the benchmark Hang Seng Index.