The £20 weekly increase in Universal Credit payments will be extended for six months, the Government has said.
Chancellor Rishi Sunak said the temporary increase, introduced at the start of the coronavirus pandemic, would remain in place for half a year and "well beyond" the end of the current national lockdown in England.
The increase has been described as a "lifeline" for struggling families.
Mr Sunak said working tax credit claimants will receive equivalent support over the next six months through a one-off payment of £500, due to the way that system works operationally.
But charities called on the Government to make the uplift permanent, saying families need "help and certainty, not a stay of execution".
Paul Noblet, head of public affairs at Centrepoint, said: "Extending the uplift for only six months does not go far enough given the ending of furlough and the increase in unemployment that we could face before Christmas.
"The pandemic may be slowing down but the economic impact continues to grow and all the indications are that young people are likely to remain the hardest hit."
Imran Hussain, Action for Children's director of policy and campaigns, said: "It makes no sense to cut this lifeline in six months when the furlough scheme will have ended and unemployment is expected to be near its highest - exactly when families will need it most. Families need help and certainty, not a stay of execution.
"There's no faster way to push more children into poverty than by snatching £20 a week out of the pockets of our country's poorest families."
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The British Psychological Society warned the six-month extension will be "very damaging" to people's mental health.
Dr Julia Faulconbridge, vice-chair of the society's Division of Clinical Psychology, said: "Psychologically, the impact of a six-month extension and the uncertainty this brings is very damaging and this decision will have a significant impact on the mental (and physical) health of those already placed at risk as a result of living in poverty.
"Increased uncertainty and worry are known to be psychological stressors, and combined with the relentless grind of trying to make ends meet, leave people just trying to survive and struggling to be the independent adults and parents that they want to be.
"People need security at this turbulent time and today the Chancellor has failed to provide that."
