The findings come before the government's first budget later this month, and with a cost of living crisis that has driven soaring use of food banks.
According to the report by a non-governmental organisation the Trussell Trust, almost a quarter of children under four are facing extreme poverty.
"Shockingly, 46 percent more children are facing hunger and hardship than two decades ago. That equates to one in five children growing up trapped in this situation," the trust said in a statement.
A UNICEF report last year found that the UK -- a G7 and NATO member, and the world's sixth biggest economy -- has one of the highest rates of child poverty among richer countries.
The Trussell Trust said that without "urgent action" from Prime Minister Keir Starmer's government the numbers facing hunger and hardship -- defined as living 25 percent below the poverty line -- would rise further.
The charity defined the poverty line as £152 ($199) a week for someone living on their own and £204 for a single parent with one child. This includes paying electricity, water and property tax bills as well as food.
Removing a two-child benefit cap -- meaning families cannot claim state subsidies for a third child born after April 2017 -- is one of the measures that could ease poverty levels, the trust said.
Labour, however, has refused to abolish it in the teeth of fierce opposition from campaigners, unions and some of its own lawmakers.
Finance minister Rachel Reeves has said she will not be able to reverse the cap in her October 30 budget due to what Labour claims is a £22 billion ($28.8 billion) black hole left by the last Conservative government.
Political battleground
Wary of accusations of economic recklessness, Labour has instead stuck to a more vague pledge to "develop an ambitious strategy to reduce child poverty" and the cap has become a political battleground.
It was cited last month in the resignation letter of the first Labour MP to quit the party since the election at which it ousted Rishi Sunak's Tories.
Rosie Duffield accused the prime minister and his top team of hypocrisy for accepting gifts of expensive clothes and hospitality from rich donors while "choosing to keep the Conservatives' two-child limit".
John McDonnell -- who served as left-wing former Labour leader Jeremy Corbyn's finance spokesman and was among seven MPs suspended by the party in July for voting for it to be scrapped -- has called it a "brutal attack on families".
"Labour in government has a moral duty to end this child suffering. It is completely affordable and will save money by preventing ill health amongst poorer children," he said.
The cost of living crisis was a major issue at the ballot box.
Over the past two year it has seen workers from across the economy from doctors and nurses to teachers and train drivers walk out over wages that have failed to keep up with inflation.
Helen Barnard, director of policy, research and impact at the Trussell Trust, said the latest figures "should not be the case in one of the richest countries in the world".
"We need urgent action on hunger in the UK because, if nothing changes, the number of people facing hunger and hardship will only increase," she added.
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