The rollout of universal credit should be put on hold, Gordon Brown is to say, warning the "cruel and vindictive" system is exacerbating child poverty. Unless plans to extend the benefit across the UK were paused or abandoned, the ex-prime minister said, it could lead to major social unrest next year.
In a speech in Edinburgh, he will also urge action in this month's Budget to boost child benefit and tax credits. Theresa May says low-income families will be protected during the rollout.
Speaking at Prime Minister's Questions, the prime minister said the expansion of Universal Credit to an extra two million people next July would initially be done on a "small-scale basis to ensure we get that right".
She appeared to guarantee that those involved, likely to include one million working families and 750,000 people unable to work because of illness or disability, would not see any reduction in their monthly income.
It was designed to make claiming benefits simpler and low-paid work more financially rewarding but its implementation, which has been phased in to different groups and regions, remains hugely controversial. The process of making universal credit available to all claimant groups across the UK has been delayed several times but is now due to go ahead in the summer.