Alexander will today voice unprecedented criticism of previous Scottish Labour leaders for letting the party lose sight of its principles and failing to reform it as Tony Blair did in England.
The new Holyrood leader will address the party conference in Bournemouth ahead of Prime Minister Gordon Brown.
Ms Alexander will not name Jack McConnell, having been careful not to antagonise him over the month between his resignation and her confirmation as his successor 10 days ago.
But the criticism of the former first minister, who is also in Bournemouth, is clear from her intended speech notes: "We will return to being a party of principle and energy - a party committed to changing itself and to changing Scotland, and a party fit to fight and fit to govern."
Ms Alexander intends to use her platform speech to warn activists from Scotland that they will have to try again in the failed attempts at party reform a decade ago.
"Back in the 1990s, Labour had to modernise to win," a late draft of the speech states. "There were a few in our party north of the border who thought modernisation wasn't for us. They were wrong.
"What Labour realised in England a decade ago, we need to face up to in Scotland today.
"To regain the trust of the Scottish people, Labour in Scotland needs to change and needs to reform."
She has said already a programme of internal reform will be put to the Scottish Labour conference early next year, but there has been little detail about what will be in it.
Much of the Scottish Labour leader's conference speech is to be aimed at SNP First Minister Alex Salmond for being "an ally" of David Cameron, the Conservative leader.
The SNP's deputy leader, Nicola Sturgeon, seized on press reports yesterday that Labour colleagues were briefing against Ms Alexander only days into her new job, with a senior spokesman on the verge of resigning in protest at her treatment of him.
Ms Sturgeon said her opponents were in disarray.
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