Unions have accused the government of "chilling callousness" against single parents and their children, as a tough new benefit cap comes into force.
The Prime Minister is unleashing an assault on struggling families that will hold back the poorest children, the GMB claimed.
The annual limit on benefits to unemployed households drops from £26,000 to £20,000 outside London, meaning 64,000 new households are hit by the cap.
However, the GMB says nearly two thirds of those affected are single mothers.
National Secretary Rehana Azam said: "Just four months ago, Theresa May stood on the steps of Downing Street promising to fight injustice and to ensure every person regardless of their background would be given the chance to be all they want to be.
"Today she is unleashing a monstrous new assault on 40,000 single mothers, which risks shattering the life chances of children up and down our country.
"This has echoes of the staggering hypocrisy and chilling callousness that saw the victimisation of single mothers in the bad old days of the early 1990s."
The cap is set at £23,000 in London for families, and £15,410 for single people without children - falling to £13,400 across the rest of the UK.
The Department for Work and Pensions says around 23,500 households who previously had benefits capped have moved into work since 2013, and added that the cap ensures work always pays.
Analysis by the Institute for Fiscal Studies suggests, in contradiction, that "the majority of those affected will not respond" by moving house or moving into work.
Work and Pensions Secretary Damian Green said: "Each statistic represents a person who has moved into employment and can now enjoy the security and dignity that works brings.
"By making sure that those people who are out of work are faced with the same choices as those who are in work, the benefit cap has been a real success.
"By lowering the cap today, we are ensuring the values of this Government continue to chime with those of ordinary working people and delivering on our commitment to make sure work pays more than welfare."