Labour and the Conservatives are coming under increasing pressure to end the two-child cap on benefits paid to families with children.
Low-income parents are currently denied key benefits, including Universal Credit, for their third and any subsequent children born from April 2017.
Research by the Institute for Fiscal Studies (IFS) finds that when fully rolled out, the policy will affect one in five children, costing families an average of £4,300 a year.
The policy already applies to about 2 million children, but this figure could rise by another 250,000 in the next year alone and 670,000 by the end of the next parliament.
Politicians across the spectrum, from Gordon Brown and leading charities to Nigel Farage and Suella Braverman, have called for an end to the ban.
Labour’s manifesto for government, published last week, included the promise of an “ambitious strategy to reduce child poverty”, but no mention of the two-child limit.
Eduin Latimer, a research economist at the IFS, said the two-child limit had “a particularly big impact on the number of children in poverty for two reasons: it mostly affects poorer households and, by definition, its effects are entirely concentrated in families with at least three children”.
Labour’s decision not to include scrapping the policy in its manifesto has frustrated charities and anti-poverty campaigners.
Last week, Keir Starmer argued that a Labour government would not make “unfunded promises”.
Rishi Sunak has also rejected calls to cancel the two child limit and has gone further with a pledge to cut £12bn from the benefits bill.
By contrast, the Liberal Democrats have promised to scrap the two-child limit as part of a package of anti-poverty measures.
The Lib Dem work and pensions spokesperson, Wendy Chamberlain, said: “It would be a devastating blow to some of the poorest families in the country if this limit remains in place. The next government must heed these fresh warnings and ensure no child is cut off from support they so desperately need.”
The IFS calculates that ending the two-child limit would cost £3.4bn a year in the long run – equivalent, it says, to freezing fuel duty for the duration of the next parliament.
Alison Garnham, the chief executive of the Child Poverty Action Group, told The Guardian that child poverty is a “national disgrace and the biggest driver of it is the two-child limit”.
She said: “It makes life worse for kids up and down the country and limits their future chances. Any government serious about making things better for the next generation will have to scrap the two-child limit, and do so quickly.”
Tom Pollard, the head of social policy at the New Economics Foundation thinktank, said: “You can’t really have a serious child poverty strategy that doesn’t involve getting rid of the two-child limit, because it just puts a ceiling on what you can do.”
