Gordon Brown is expected to make an eleventh hour intervention in the Labour leadership race against a backdrop of Jeremy Corbyn's soaring popularity.
The former Prime Minister has yet to reveal who he is backing in the race but a Sky News report suggests he will endorse Yvette Cooper.
Gordon Brown's comments ahead of the Scottish referendum was seen as crucial
His predecessor, Tony Blair, has been outspoken over the prospect of left-winger Mr Corbyn becoming leader, warning of the party "walking off a cliff". A YouGov poll gave the Islington North MP a 32-point lead.
Those fearful of a Corbyn victory may hope the former MP sounding an alarm will dent his prospects more than Mr Blair, whose comments appear to have emboldened "Corbyn-ites".
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Mr Brown's public comments on Scottish independence just days before last year's referendum were credited for shoring up the "No" vote against a shock poll suggesting a victory for the "Yes" campaign.
His speech in south London on Saturday is entitled "Power For A Purpose – The Future Of The Labour Party" and comes as party officials began sending out more than 600,000 ballot papers to members.
Labour leadership contender Jeremy Corbyn takes part in a campaign rally in Edinburgh
Mr Corbyn is today holding a rally in Scotland to outline his 10 priorities if elected Prime Minister. Ed Miliband has yet to make any public remarks on the race.