Former US secretary of state Henry Kissinger has resigned as chairman of a panel investigating the 9/11 attacks, citing controversy over potential conflicts of interest with his private-sector clients.

"Although specific potential conflicts can be resolved, the controversy would quickly move to the consulting firm I have built and own," Kissinger wrote in a letter to President George W Bush.

"I have, therefore, concluded that I cannot accept the  responsibility you proposed."

Bush issued a written statement, saying he accepted  Kissinger's resignation with regret.

The decision was a serious blow to the panel. It's original vice-chairman, former senate majority leader George Mitchell, resigned from the commission on Wednesday, partly because of pressures to quit his law firm.

Kissinger's resignation came one day after he tried to assure 9/11 victims that his business interests would not conflict with his duties as chairman of the panel.

The White House and Congressional Democrats had clashed on whether he had to disclose his business clients, with Bush's advisers saying the law did not require such disclosures.

Kissinger had come under strong criticism, both from the liberal media and some of the families of the victims, because he refused to name the clients of his consulting firm, Kissinger Associates, to assure the public that there was no conflict of interest.

Another reason, according to some analysts, is the question whether among his clients is anyone whose country has been funding the terrorists.