Around 120,000 children will be cast adrift if the UK government changes its definition of child poverty, social justice secretary Alex Neil has claimed.

In a letter to the secretary of state for work and pensions Iain Duncan Smith, Mr Neil urged the UK government to focus on tackling the root causes of child poverty instead of redefining the way it is measured.

A new way of measuring child poverty was announced by Mr Duncan Smith earlier this month.

The Scottish Government said the new definition would mean the UK government would no longer be required to take action to reduce the number of people living on low incomes.

Instead, the focus would move to “worklessness” and “educational attainment”.

UK government figures, released in June, showed an increase in the number of children living in poverty to 3.71 million after housing costs were taken into account. Of this total 65 per cent of the children were from families where at least one parent was working. This highlights the fact that progress in reducing child poverty, and improving the chances for all children, will be difficult without improvements to the living standards of working families. However these figures will not be taken into account if the proposed Welfare Reform and Work Bill is passed.

Mr Neil said: “By changing the definition of child poverty the UK government is hiding the true extent of the problem and casting adrift the 120,000 Scottish children whose parents are working on low incomes and struggling to pay their bills.

“The secretary of state for work and pensions must rethink these flawed plans. They will only gloss over the impact of the UK government’s austerity agenda and fail to show the shocking reality of its inexcusable attack on low-paid families.

“The Scottish Government will continue to measure and report on the wide range of factors that drive child poverty including income, educational attainment and health outcomes. Our sophisticated measurement framework was developed with experts and leading children’s organisations and is helping us to understand the full scale of the problem and find the most effective ways to address it.

“Around 210,000 children are living in relative poverty after housing costs are paid, but these numbers are likely to soar in coming years because of cuts to social security. Reforms to tax credits alone will reduce the incomes of between 200,000 and 250,000 households in Scotland, with families facing almost £700m of cuts.

“We recognise that any serious attempt to tackle inequality has to focus on in-work poverty, which remains very high. That’s why we are calling for powers over the minimum wage, employment policy and working-age benefits to be devolved to Scotland.

“We have invested £296m in welfare measures and around £329m over two years to expand free early learning and childcare, including extra provision for disadvantaged children, while our work to encourage employers to pay the Living Wage is also helping to increase income levels in Scotland. We have appointed our first Independent Adviser on Poverty and Inequality who will be looking at what more we can do to address inequalities.”

A Department for Work and Pensions spokesman said: “Eradicating child poverty is an absolute priority for this government, and we have consistently argued that it is not enough to tackle the symptoms without also tackling the underlying causes.

“Our new approach will drive effective government action by focusing attention on making meaningful change to children’s life chances.”