The government has been accused of disguising the number of children in poor families with a "sleight of hand".
Campaign group End Child Poverty, says changes to the way the figures are calculated appear to cut the total from 3.8 million to 2.7 million.
Previously, calculations to measure poverty did include housing costs - as well as income.
However, the government says the change is to bring the UK into line with international poverty statistics
Counting the poor has been a hot topic since the prime minister promised to end child poverty within 20 years.
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The news comes as spending watchdog the National Audit Office says the battle to beat poverty is showing signs of success.
Over the next 10 years, £2bn is being ploughed into England's most deprived areas under the anti-poverty New Deal for Communities scheme.
To measure its progress in battling poverty, the government is switching to lower measure of counting the incomes of poor families first before their housing costs have been taken into account.
The Department of Work and Pensions says this brings the UK into line with global statistics which puts the number of poor children in the UK at around three million.
But, the move has angered anti-poverty campaigners as it ignores the high cost of housing.
They say it ignores the people who are officially poor after they have paid for rent or mortgages, especially in London or the South East.