Former federal minister Warren Snowdon is supporting Bill Shorten to become Labor's new parliamentary leader.
Mr Snowdon, who hails from the so-called "soft Left" faction, was expected to favour the left's leadership candidate Anthony Albanese.
But a spokesman told AAP on Friday the former veterans' affairs minister would be backing Mr Shorten, from the right faction.
Mr Snowdon's support is predicated on former ministerial colleague and left faction member Tanya Plibersek becoming Mr Shorten's deputy.
His decision is a sign left MPs in caucus have fractured over the leadership, which will for the first time be determined by a ballot evenly weighted between the caucus and grassroots ALP members.
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The ballot opens on Tuesday and closes on October 9.
The caucus meets on October 10 to cast its vote for the new leader and the winner will be announced on October 13.
Mr Albanese has strong popular support among ordinary ALP members, but is understood to now have only about 40 per cent of the caucus behind him.
