JUST days after Treasurer Joe Hockey denied a takeover bid for Australia's largest grain handler, Graincorp, chief executive Alison Watkins announced she would resign.

In a statement to the stock exchange, Ms Watkins said she would leave the company at the end of January next year.

Her departure was unexpected so soon after the takeover proposal by United States giant Archer Daniels Midlands was denied.

Ms Watkins, who had been promoting the takeover to shareholders, said her decision was "enormously difficult" and made with "great sadness".

Chairman Don Taylor said an internal and external search for a new CEO was already underway, with Mr Taylor to act as executive chairman in the interim.

Mr Hockey's decision has been slammed by the business community as a political move, after a public campaign by some members of the Coalition against the takeover bid.