Save The Children described the figure as a "national scandal" and claims the number is likely to grow as Government cuts to welfare and public services bite.
It is calling on Chancellor George Osborne to counter the recent increase in VAT and rising inflation and unemployment with an emergency plan in next month's budget. Save The Children says even more families will struggle to get by unless more jobs are channelled to the worst-off areas and more financial help is given to parents who want to work.
Its research, carried out by New Policy Institute, found that 29 local authorities in Britain have more than one in five children living in severe poverty.