The government's Households Below Average Income statistics define child poverty as children living in homes with 60% less than the median UK income. The median is the middle figure in a set of numbers.
The current government, however, believes the measure of poverty is too narrow and can be perverse because, if average incomes fall, the poverty line falls too.
The level of household income which defines "in poverty" fell from £259 a week, in 2009-2010, to £251 the following year.
'Life chances'
The BBC's Mark Easton said that drop explained why 300,000 fewer children were classed as living in poverty.
Mr Duncan Smith told BBC Radio 4's Today programme that as middle and upper incomes rose from 2003 onwards, Labour ministers went on "a massive spending spree on welfare" to ensure that those at the opposite end of the spectrum did not fall below the poverty line.
After housing costs were taken into consideration, the number of children living in relative poverty was 3.6 million (27%) - down 2% on the previous year.
He said that while income would "always play a part in future measurements of poverty", focusing on money alone resulted in people becoming trapped on benefits.
The government, meanwhile, believes the current measure of poverty is too narrow and can be perverse because, if average incomes fall, the poverty line falls too.
"It's not that redistributing income [isn't] right... we have to do some of that, but we need to make sure that what we're doing has a change to someone's life," he said
Work and Pensions Secretary Iain Duncan Smith told BBC Radio 4's Today programme that while income would "always play a part in future measurements of poverty", focusing on money alone resulted in people becoming trapped on benefits.
"Not just leaving them as they are with more money, but actually getting them to the state... where they take control of their lives.
"That's fair to them and then that becomes fair to taxpayers who accept that what they're paying their taxes for is reasonable because they're seeing something changing."
In a speech later, Mr Duncan Smith will cite the example of drug-addicted parents who may be technically taken out of poverty by an extra pound but spend that money on another drug fix.
In a speech later, Mr Duncan Smith will cite the example of drug-addicted parents who may be technically taken out of poverty by an extra pound but spend that money on another drug fix.
"Unless we find a way of properly measuring changes to children's life chances, rather than the present measurement of income alone, we risk repeating the failures of the past," he will say.
"Unless we find a way of properly measuring changes to children's life chances, rather than the present measurement of income alone, we risk repeating the failures of the past," he will say.
But Alison Garnham, from the Child Poverty Action Group, said there were "mountains of evidence" which showed that "low income is associated with significant deficits" in children's lives.
"Poor children on a low income are behind in education, they are likely to suffer more lifelong limiting illnesses, die younger, and have poor self-esteem," she told the BBC.
Universal credit
Mr Duncan Smith will present figures suggesting the government's universal credit - which will replace a series of benefits and tax credits - will pull the "vast majority" of young people out of poverty if at least one parent works 35 hours a week at the minimum wage. The figure would be 24 hours for a lone parent.
But Oxfam's director of UK poverty Chris Jones said 60% of working-age adults in poverty were not from jobless households.
"The government is justifying huge cuts to welfare support for people on low incomes by saying this will incentivise work, but there simply aren't enough decent jobs available," he said.
"If we carry on down this path, the UK will return to levels of inequality not seen since Victorian times."
Labour's shadow work and pensions secretary Liam Byrne said: "In a great country like Britain we should stand behind parents fighting to bring up their children free of the curse of poverty.
"Behind [Prime Minister David] Cameron's promises we learn today that those parents and their children will now be abandoned and told, 'you are on your own'."
A consultation on how best to measure child poverty will begin in the autumn.
Do you consider yourself or your family to be living in poverty? Do you think the definition of child poverty needs to change? If you are willing to be interviewed by the BBC please fill in the form below.
Do you consider yourself or your family to be living in poverty? Do you think the definition of child poverty needs to change? If you are willing to be interviewed by the BBC please fill in the form below.
