Abbott backs commission to proceed without Heydon

PRIME Minister Tony Abbott has vowed the Royal Commission into Trade Union Governance and Corruption will not stop even if Justice Dyson Heydon leaves the investigation.

Justice Heydon had been expected to decide his future on Tuesday after union submissions last week arguing he should recuse himself for the appearance of bias, or bias itself, but on Monday said he was still considering his position.

Mr Abbott told reporters in far north Queensland the inquiry "must" and would go on even if Justice Heydon stepped down.

Mr Abbott cited comments from former union leader Martin Ferguson that the inquiry was "important for the union movement" and the Labor Party.

The controversy follows Justice Heydon's acceptance of an invitation to speak at a Liberal Party fundraiser.

He subsequently declined to appear at the function when news of the engagement broke, and said he had not realised it was a Liberal Party function.

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Mr Ferguson told the ABC's Four Corners program on Monday night the inquiry was not a "political plaything" and refused to damn it, putting him at odds with many figures in the union movement.

On Tuesday, Labor leader Bill Shorten rebuffed Mr Ferguson's defence of the inquiry and described it as a political witch-hunt.