The National Audit Office pointed to a huge rise in demand since 2010.

Council budgets had been cut by nearly 30% over this period, it said, and 91% of local authorities were now overspending on children's services.

The Department for Education said it was working to improve its understanding of the growing pressures.

The NAO report, Pressures on Children's Social Care, said the number of children taken into care had trebled over the period, once population growth had been accounted for.

Children's homes

And the number of child-protection assessments made by local authorities had risen by 77%.

Overall, local authorities were planning to spend £350m, or 9%, more in 2018-19 than they had budgeted for during the previous financial year, the NAO said.

Meg Hillier, who chairs the Commons Public Accounts Committee, which scrutinises NAO reports, said: "With an overspend of £872m last year on children's social care and with local government under such funding pressure, government has got to grasp the nettle.

"Children's safety and wellbeing must not be subject to a postcode lottery and it is appalling that the department does not fully understand what is driving demand for children's social care or why there are such wide variations between local authorities."

One of the key pressures on children's services since 2010 has been the rise in the number of the most serious and expensive cases - children taken into care.

Little analysis

At the same time, there has been an increase in the use of residential care, ie children being cared for in children's homes.

However, only just under a third of local authorities say they have access to enough residential homes for children aged 14 to 15, and four out of 10 for those aged 16 and 17.

An independent review of capacity found an absence of successful commissioning was resulting in different local authorities "paying wildly different prices for the same standard of residential care".

And as well as boosting social worker numbers, local authorities have had to increase the use of agency staff, according to the NAO.

It said: "The Department [for Education] does not fully understand what is causing increases in demand and activity in children's social care."

Until recently, the DfE had not seen this as a central part of its responsibilities, the NAO said.

"As a result, it had little quantified analysis of the drivers of demand or reasons for variation between authorities."

'Not efficient'

Ministers and officials blame the rise in demand on:

• deprivation
• domestic abuse
• substance misuse
• poor adult mental health

But the NAO called their understanding "analytically limited" and "not comprehensive".

The Local Government Association said: "It is clear that the most urgent and pressing issue is not variability but the very real funding crisis facing vital children's services across the country, which face a funding gap of £3.1 bn by 2025."

Jonathan Stanley, chief executive of the Independent Children's Homes Association, said: "Another year, another NAO report pointing inextricably one way to the strategic planning that is essential; co-production by providers and local authorities has to be directed.

"The policy direction of the DfE heads in the opposite direction, away from any government responsibility or involvement, by devolving powers and practice to local authorities.

Children's Minister Nadhim Zahawi said: "As the report acknowledges, we are currently working across government to improve our understanding of demand for children's services.

"And we know there are pressures on councils, which is why we are providing an additional £410m in the Budget for adult and children's social care and an extra £84m to expand innovative practice to support vulnerable families across a further 20 councils."

Source: BBC