Stripping hard-up families of £20 a week next April would be “unconscionable”, a former Tory minister warned Boris Johnson.
The Government announced a rise in Universal Credit to help households on the benefit through the coronavirus crisis.
But the temporary hike is due to end in five months.
Former Work and Pensions Secretary Stephen Crabb warned the fact it is a temporary increase is in "the small print".
He told BBC Radio 4's Today programme: "We are on course to cut that money back in just a few months time and for me that's just unconscionable.
"You can't give money to some of the poorest people in the country and then go out publicly and celebrate that fact and then say 'oh by the way, even though we may still be living with the consequences of the pandemic next April we are going to cut that money back by around £20 per week, £100 per month' – and that's going to be a much bigger headache for the Government than school meals."
His warning comes as Boris Johnson came under pressure from all sides over the failure to extend free school meals during the holidays.
Ministers are considering using holiday clubs to feed hungry children in an attempt to defuse a damaging political row which exploded after Tory MPs blocked a bid led by footballer Marcus Rashford to extend the scheme during the holidays.
But the Prime Minister's own food tsar said the Government "isn't doing enough" to address child hunger.
Henry Dimbleby, co-founder of Leon, said: "This problem is real. It should go without saying it's serious. It's immediate and it's going to get worse as employment gets worse and the Government isn't doing enough.
"One in seven families already are reporting not being able to afford enough food."
He said that "in-kind support" through holiday clubs, providing food and education, had been shown to have a better impact than putting the same "small amount of money" into Universal Credit.
"I haven't been backward in coming forward with ideas that I have been feeding in to Treasury, to (the Department for) Education and to Number 10, ideas of how they could rapidly implement this by Christmas," he said.
"But ... the dark centre of government is invisible to me and I have no idea exactly what they're working on as we speak."
Mr Dimbleby's Holiday Activity and Food Programme was trialled across 17 local authorities over the summer.
Business Minister Nadhim Zahawi said the pilots were being looked at.
"The best way to do it - as the Prime Minister quite rightly outlined - is through local government. 17 local authorities participated in that pilot and of course through the actual welfare system, the Universal Credit system, delivering that additional help for those families," he told Sky News.
