Craig Emerson will act as Australia's Foreign Minister after the shock resignation of Mr Rudd.
Dr Emerson, presently the Trade Minister, will take on Foreign Affairs responsibility immediately.
But a senior Labor source said decision on a permanent replacement for Mr Rudd is not expected for some time.
Prime Minister Julia Gillard now must decide whether to reshuffle her front bench as anticipation builds that Mr Rudd will challenge for the top job.
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