Prime Minister Sir Keir Starmer has criticised how the Office for Budget Responsibility watchdog has analysed the impact of his benefit changes.
Appearing at a parliamentary committee, he said the OBR, which monitors public spending plans, had assumed that "not a single person" would change their behaviour as a result of the government's proposals.
"I personally struggle with that way of looking at it, because I do think these measures will make a material difference," Sir Keir said.
The prime minister hopes his plans, which include making it harder to claim disability payments, will save money and help get more people into work, however there is uneasiness among his own MPs over the cuts.
Sir Keir was challenged over the plans during an appearance in front of Parliament's Liaison Committee, made up of senior MPs.
Debbie Abrahams, chairwoman of the work and pensions committee, noted that an impact assessment produced by the government said the changes could push 250,000 people into poverty, while the Joseph Rowntree Foundation charity put the figure at 400,000.
The Labour MP said children who grow up in poverty are more likely to struggle to find work or employment later in life and asked the prime minister how he would ensure his approach addressed the root causes of poverty "rather than add to them".
