A Malaysia Airlines flight to Japan has been grounded after maintenance crew found hydraulic oil spilled on the instrument panel in the cockpit, a report said today.

It was the flag carrier's fourth aircraft to be grounded under "unusual circumstances" in the past two years, The Star newspaper said, quoting sources. Previous cases involved electrical wires that had been tampered with.

The daily said a Malaysia Airlines Airbus A330, which arrived at Kuala Lumpur International Airport from Bangkok on Wednesday, was scheduled to depart for Osaka later that day.

However, the aircraft, which was parked less than 10 metres from a security post, was immediately grounded after engineers found an oily substance on the cockpit instrument panel and a small can during a pre-departure inspection, it said.

The 179 passengers departed on another aircraft after a 35-minute delay.

Sources told the daily that initial investigations revealed that the carrier did not use the brand of hydraulic oil found.

Malaysia Airlines, in a statement, confirmed that engineering maintenance crew did find the oil during a routine inspection and said it was working closely with police.

"Malaysia Airlines has stringent policy and procedures to ensure the safety of its aircraft and passengers at all times and there will be no compromise," it said, with elaborating.

A police spokesman said investigations were being carried out but gave no details.