(Reuters) -Singapore Post said on Sunday it had dismissed its chief executive and two other senior managers after an internal investigation found they had mishandled whistleblower allegations about misconduct by company employees.

The firm's shares slumped by the most in over four years as the city-state's bourse opened for trading. The stock plunged as much as 9.8%, its biggest intraday loss since March 2020.

The alleged misconduct reported by the whistleblower related to several employees who worked in the company's international e-commerce logistics parcels business, the company said in a statement, without giving further details.

The firm had received a whistleblowing report alleging that three unnamed employees had either approved or manually updated a "delivery failure" status code for parcels, even though no delivery was done, the statement said.

CEO Vincent Phang, the chief of the company's international business unit Li Yu, and Chief Financial Officer Vincent Yik had been dismissed after the company found they had been "negligent" in handling the case and misrepresented it before an audit committee, it added.

Chair Simon Israel will oversee senior management of Singapore Post in the interim. The company said it planned to announce a new CEO in due course.

Singapore Post also said it will pay a settlement to the customer, whose details the firm did not disclose.

Phang and Yik have indicated they will "vigorously contest the termination of his employment, both on merits and on the grounds of procedural unfairness," the company said.

The firm has been undergoing a strategic review under which it is selling Australian assets to private equity Pacific Equity Partners for A$1.02 billion ($638.21 million).

($1 = 1.5982 Australian dollars)