The four UK children's commissioners have joined forces to urge the Government to halt its programme of benefit cuts to prevent more young people being pushed into poverty.
The commissioners for England, Scotland, Wales and Northern Ireland said child poverty rates across the UK were "unacceptably high" and that there had been a failure to protect those from the most disadvantaged backgrounds.
In a joint report to the United Nations, the commissioners also voiced concern at the impact on children of the Government's plans to scrap the Human Rights Act (HRA) - which enshrines the European Convention on Human Rights (ECHR) in UK law - and replace it with a British bill of rights.
"The HRA has been vital in promoting and protecting the rights of children in the United Kingdom and the European Court of Human Rights has had an important role in developing the protection offered to children by the ECHR," it said.
"The commissioners are concerned that any amendment or replacement of the HRA is likely to be regressive."
Their intervention - just a week before Chancellor George Osborne delivers a Budget in which he is expected to set out further details of the Government's plans to cut another £12 billion from the welfare bill – is likely to anger ministers.
In their report reviewing the past seven years, the commissioners said the Government's "austerity" policies adopted in response to the global financial crisis had resulted in "a failure to protect the most disadvantaged children and those in especially vulnerable groups from child poverty".
"The best interests of children were not central to the development of these policies and children's views were not sought," the report said.
"Reductions to household income for poorer children as a result of tax, transfer and social security benefit changes have led to food and fuel poverty, and the sharply increased use of crisis food bank provision by families.
"In some parts of the UK there is insufficient affordable decent housing which has led to poorer children living in inadequate housing and in temporary accommodation."
The commissioners also highlighted concerns over failures to tackle child abuse, the treatment of young people in the criminal justice system and the provision of mental health services for children and young people which they said were "vastly underfunded".
They pointed to a recent forecast from the Institute for Fiscal Studies suggesting the number of children living in poverty could more than double from 2.3 million to 4.7 million by 2020.
They said they were "deeply concerned" by reports the Government was now planning to scrap the Child Poverty Act which requires ministers to put in place measures to eradicate child poverty.
Scotland's commissioner for children and young people, Tam Baillie, said the Government now needed to re-think its austerity plans for the sake of children across the UK.
"It is deeply disturbing that the UK Government, aware of the current and future impact of its cuts, appears to be targeting the most vulnerable people in our society," he said.
"The UK Government's austerity measures have condemned 2.3 million children into poverty and that number will increase if further proposed cuts are enacted.
"For one of the richest countries in the world, this is a policy of choice and it is a disgrace. It is avoidable and unacceptable."
The other commissioners are Anne Longfield, the commissioner for England; Sally Holland, the commissioner for Wales: and Koulla Yiasouma, the commissioner for Northern Ireland.
A Department for Work and Pensions spokesman said: "This report ignores the fact that the proportion of individuals in relative poverty is now at the lowest level since the mid-1980s.
"The best route out of poverty is work and this Government makes no apology for its efforts to raise incomes by expanding employment opportunities.
"Our reforms to the welfare system are focused on making work pay and our reforms to the tax system are allowing people to keep more of what they earn."
The chief executive of the Child Poverty Action Group, Alison Garnham, described the commissioners' report as "a significant intervention ... which cannot be ignored".
Ms Garnham said: "They now join the Supreme Court in expressing unease about how key austerity policies are affecting children. We cannot be claiming to be reducing the deficit in the name of our children if we're choosing to do that through policies that damage children's lives and prospects.
"We will have a bigger child poverty crisis on our hands if there are further cuts to support for low income families and the low-paid. Instead of spending billions on the personal tax allowance - which does least for the lowest-paid families - the Government should be shielding children's benefits with the same triple lock protection that pensions have."
In a separate report, the Children's Rights Alliance for England claimed that the Government was breaching its obligations under the UN Convention on the Rights of the Child (CRC), because it was not fully considering how its decisions affect children.
The CRAE report said that Government policies meant continued poverty for millions of children and a reduction in spending on services for families at a time of increasing need.
Many vulnerable children were no longer entitled to help with legal advice and representation and child migrants are facing more restrictive policies, said the report, which put forward a series of recommendations to make children less "invisible" in policy decisions.
It cited as examples of threats to children's rights the use of restraint holds on children in detention, an increase in the use of Tasers on young people and the UK's position as the only European country to recruit 16-year-olds to the armed forces.
CRAE co-director Louise King said: "Every child has a human right to a good start in life – an equal chance to do well at school, to be healthy, safe and cared for.
"But too many children are invisible to the Government. Decisions on welfare, cuts to vital services, and children's day-to-day treatment all threaten children's human rights."
Peter Grigg, director of external affairs at The Children's Society, said: "As we see from our frontline work every day, Government measures are pushing many migrant children into poverty, homelessness and exploitation. We urge the Government to take urgent action to reverse this trend by following its commitments under the UN Convention on the Rights of the Child. Migrant children are children in need - not immigration statistics."
Kate Mulley, director of policy and campaigns at Action for Children, said: "We work with some of the country's most vulnerable children and their families. Our experience of getting help and support to families early can prevent situations from reaching crisis point. Spending on preventing harm to vulnerable children, rather than picking up the pieces when it's too late would change lives and save millions in taxpayers money."