TD Ameritrade one of the biggest investors in Knight Capital, pledged its support for the brokerage firm's acquisition by rival Getco, in the latest sign that the deal is likely to win shareholder approval.

The backing for the deal from TD Ameritrade, an online-brokerage giant, adds momentum to Getco's $1.4bn bid for Knight, after Knight's biggest shareholder, investment bank Jefferies, also signalled support for the deal last week.