Gordon Brown is to claim removing Boris Johnson from Number 10 is the first step in restoring "common decency" and moving the UK forward.
The former prime minister will be speaking at three events across Scotland - in Fife, South Lanarkshire and East Lothian - on Saturday amid the General Election campaign.
He will highlight child poverty as an issue which should be central to concerns of politicians and voters on December 12, with 121,925 children in working families north of the border living in such conditions.
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Mr Brown is to say: "Boris Johnson's departure from Downing Street on December 12 is the first step to moving our United Kingdom forward - united again in purpose, compassion and common decency.
"People talk of the government spending more but austerity has not ended for millions of families with children and millions of low-paid workers.
"Austerity for them will continue throughout the next parliament if Boris Johnson's policies are implemented.
"The UK is already torn apart by Brexit, divisive nationalisms, a north-south divide and the crude trolling of vulnerable people of all sides.
"We are so disunited we have lost sight of a once-compassionate, state-guaranteed safety net and seem unable to work together - even to come to the aid of vulnerable children.
"Labour is absolutely right to say it will abolish universal credit. This discredited system - the brainchild of Iain Duncan Smith - is chaotic, demeaning and poverty-inducing. Nothing can make it work.
"The first step to relieving child poverty is the end of universal credit.
"A caring nation would increase child benefit substantially, raise the child credit, end the withdrawal of benefits from three and four-child families and guarantee that there is always enough housing benefit in place to pay rent."