Alinta senior management had been banking on being able to control AGL with only 35% of shares, as voting patterns at AGL meetings over the last few years indicate that few of its 110,000 retail shareholders are active in company affairs.

However, the Takeovers Panel has ruled that both companies must include conditions in their offers requiring them to acquire more than 50% before the offers can become unconditional. The conditions cannot be waived without the panel’s consent.

The panel aims to ensure that neither Alinta’s nor AGL’s competing hostile takeover offers can become unconditional until it is clear that one bid has been successful and the other has not.

Its orders will mean the companies will have to build a stake of 50.1% in order to complete a takeover.

“The panel has also required that the successful offer be open for at least two weeks after it becomes unconditional in order to allow shareholders the time to reassess their positions once control of the two companies has been decided,” the panel said in its ruling.

In February, Perth-based Alinta made an $8.9 billion bid for larger east coast rival AGL, through a merger proposal that would create a combined group worth almost $12 billion.

In March, AGL counter-attacked with its own takeover bid for Alinta.

AGL has welcomed the decision but Alinta has said it intends to make an application for review of the ruling.