A battle between the New York Stock Exchange (NYSE) and its largest specialist trading firm LaBranche has deepened, following the resignation of LaBranche's chief executive as vice chairman of the NYSE.
LaBranche is one of five firms being investigated by the Securities and Exchange Commission (SEC) over alleged trading violations.
But the group refused to hand over emails demanded by the NYSE as part of its inquiry.
Now chief executive Robert Murphy has resigned from the exchange's board, to "devote his full attention to his duties" at LaBranche, the firm said in a statement.
The exchange is reviewing the trades of some of its specialists - traders on the NYSE floor who manage the buying and selling of shares - to check for any "undue" dealer intervention.
The move follows a warning from the SEC and the New York Attorney General Eliot Spitzer that the NYSE was expected to clean up its corporate governance and bring itself up to the standards of a publicly quoted company.
As part of the investigation, the NYSE has asked specialist firms to provide it with old e-mails.
LaBranche said its employees had handed over numerous interviews and thousands of emails but drew the line at those it claimed were personal and not relevant to the investigation.
The exchange said no pressure was put on Mr Murphy to resign and that he had decided to leave of his own account.
But it now means LaBranche is the only company at the centre of the inquiry to be without board representation.
The other four firms being asked to comply with the inquiry are Fleet Financial, Van Der Moolen, Bear Wagner and Spear, Leeds & Kellogg.