Britain is trailing behind many countries in the developed world by failing to reduce inequality in child well-being, according to a new report by Unicef.
The new study, named Report Card 13, shows that Britain is struggling to tackle gaps in child health, education and income inequality, which is helping to create a growing divide between children from wealthy and disadvantaged backgrounds.
Among the key areas of concern, Britain was ranked bottom out of 37 countries for the disparities in healthy eating between children from differing social and financial backgrounds.
The report also reveals that British children suffer from one of the largest gaps in the levels of physical activity enjoyed by different socio-economic groupings.
Britain is ranked 25th overall for education outcomes, with as many as one in 10 students failing to meet minimum proficiency levels in reading, science and maths.
The findings reveal that British children from disadvantaged backgrounds are now lagging behind Slovenia, Poland and Romania for their ability in the three key subjects.
Lily Caprani, Unicef UK's deputy executive director, said the report's findings show that Government ambitions to eradicate child poverty in the UK are unlikely to materialise in the near future.
"We must be more ambitious for our children, Britain can and must do better," she said.
"The UK Government should adopt, as a matter of urgency, a childhood obesity strategy that promotes and supports healthy lifestyles for low-income children.
"It must also act to further reduce income inequality, which includes protecting social transfers."
Denmark was ranked first in the report's findings, while Israel and Turkey were found to have the highest levels of child inequality.
The UK was ranked 14th overall, tied with Germany, Greece and Hungary.
Children's Commissioner Anne Longfield said: "Children and young people in England have great opportunities, but Unicef's report highlights the difficulties faced by up to a million children who continue to fare worse than the majority.
"We know from other research that there has been progress in some areas - rates of teen pregnancy and alcohol consumption have decreased - but it is critical struggling families and children get the support they need, particularly in early childhood, before problems take hold.
"That is why I'm backing proposals for an early years indicator to measure disadvantage and why I'm encouraging local authorities to put innovation at the forefront and revolutionise the support provided to the most vulnerable children by applying for a share of the recently announced Government funding."
A Government spokesperson said: "We're working to eliminate child poverty and improving life chances for children and there are now 300,000 fewer children in poverty.
"We know there's more we can do and that's why we've introduced the National Living Wage which is increasing the incomes of the lowest paid.
"All infant pupils can now get free school meals - meaning 1.3 million more children get a nutritious free meal at lunchtime, saving families hundreds of pounds. And we continue to spend £80 billion a year to provide a safety net for those who need extra support."