The government has pledged to eradicate child poverty by 2020
An extra £4.2bn a year will have to be spent on tax credits if the government is to meet its target of halving child poverty, a report warns.
The Joseph Rowntree Foundation (JRF) estimates that 2.3 million children will be in poverty in 2010, missing the 1.7 million target set in 1999.
Treasury Secretary Yvette Cooper said tackling poverty was "about more than financial support".
The Tories said ministers were failing to address the root causes of poverty.
The government aims to eradicate child poverty completely by 2020.
But Shadow Work and Pensions Secretary Theresa May said Labour's "one-dimensional approach" to the problem had failed.
"Simply relying on means-tested benefits to address the symptoms of poverty is an unsustainable approach," she said.
"Instead we must tackle the root causes of poverty, such as educational failure, family breakdown, drug abuse, indebtedness and crime."
The Rowntree report suggests that the recession may not increase the number of children living in poverty, but could push many further below the poverty line owing to increased unemployment.
It warns: "Overall, it is possible that recession will bring a net increase in children's hardship even though it does not raise the child poverty total.
"This is likely to raise the cost of tackling child poverty, since it is more difficult to lift children out of severe poverty."
Research carried out by the Institute of Fiscal Studies found that the child element of the child tax credit would need to be raised by £12.50 a week to meet the target, adding up to £4.2bn a year.
It added that if the 2010 target was missed, "this will make it more difficult to meet the more ambitious goal of eradicating child poverty by 2020".
Child poverty could even rise to 3.1 million people by 2020, without any new policies to help low-income families, the report warned.
It stood at 3.4 million when the targets were set by Tony Blair's government.
Report author Donald Hirsch said: "The challenge in a recession will be to build on the progress already made in reducing child poverty.
"But it is unrealistic to assume that planned welfare-to-work measures will bring large increases in the number of parents with jobs, as in previous projections.
"During the recession, those families who remain out of work will need extra money if they are to avoid severe poverty, which can do irreparable harm to children who grow up in such circumstances."
Ms Cooper said the government had already prevented hundreds of thousands of children growing up in poverty during the last decade.
"It's right to be ambitious and our targets are about the kind of society we want and the future we want to build for our children," she said,
"Preventing child poverty is about more than financial support - it includes services from Sure Start to Decent Homes to give families a better start in life."
Kate Green, from the charity Child Poverty Action Group, said giving more money via tax credits would make "an incredibly important difference" to poor families.
"First of all, it gets there very quickly," she told the BBC.
"It would enable them to avoid the sort of material hardship we can expect in the recession. It would help families to pay bills or enable kids to have the odd treat at the weekend.
"In these hard times it will make a big difference to quality of life."
Ms Green said she believed the 2020 target would be reached and the recession could actually provide an opportunity to fundamentally change Britain's economic model.
"We know the public are very concerned about the kind of inequality that got us into the economic mess that we're in - the soar-away bonuses for rich bankers," she said.
"What we have a chance to do now is really reshape the whole economic balance between rich and poor."