Families with vulnerable youngsters are unable to get help from cash-strapped councils, the children’s commissioner for England has said.

Anne Longfield’s comments followed Tory-run Northamptonshire and East Sussex councils’ plans for major reductions in the services they provide across all areas of activity as a result of funding shortfalls.

Longfield said she had written to ministers calling on the government to intervene to ensure youngsters across the country were protected from local authorities’ financial difficulties.

“I’m extremely worried that the financial difficulties that Northamptonshire county council are facing will mean that they are not going to be protecting the services for the most vulnerable children, which could have catastrophic consequences for those children,” she said.

Work carried out with the Institute for Fiscal Studies showed “half of all the spending on children’s services goes on the 70,000 children who are in care”, she said, and “if you add in those who are on the child protection registers, that’s over 80%, leaving very, very little for any others”.

She told BBC Radio 4’s Today programme on Saturday: “Councils have been warning for some time that they are not going to be able to meet their statutory requirements.

“I can see and hear every day from families and children who simply can’t get help.”

There were about 1.5 million children living in families with very high needs such as “severe mental health problems, domestic violence in the household” who were not getting “any form of substantial help”, she said.

“If you don’t help children when the problems aren’t at crisis point then the crisis is going to be developing and also it is going to be much more costly when it gets to that point.”

The money in the system had remained the same for 10 years but was now being spent on “fewer children who are costing more because it is at crisis point”, she said, and pointed out that the Local Government Association had warned of a £2bn black hole.

The Tory MP Chris Philp, a ministerial aide to the communities secretary, James Brokenshire, insisted councils were getting more money.

“Of course we have had to save money as a country because we had an enormous deficit eight or nine years ago and we haven’t completely got rid of it,” he told Today. “But one of the areas that local councils across the country have quite rightly protected are children’s services.

“If you look at the amount of money being spent on children’s services, the National Audit Office reported in March this year that actually real terms spending on children’s services has gone up by about 3% across the country, so councils have very sensibly and wisely particularly protected children’s services, and that’s definitely the right thing to do.”

Over the next two financial years “the spending power that local authorities have in real terms is going to go up by about £2bn”, he said.

The Department for Education said: “Nothing is more important than ensuring children are kept safe. Northamptonshire county council – like all councils – have statutory duties ‎towards children which they must fulfil. We are working across government to ensure these duties are met, while the council seeks financial stability.

“We are already supporting Northamptonshire through our innovation programme to improve the quality of its children’s social care services.”

The shadow communities secretary, Andrew Gwynne, said: “It is a national scandal that vulnerable children are paying the price for Tory austerity. Austerity has hit some of the most vulnerable in our society the hardest. Many local councils are doing all they can, but unprecedented funding cuts have made it impossible to continue delivering services that many depend on.”

The chief executive of Barnardo’s, Javed Khan, said: “It’s vital funding is found to provide essential early support services for families. If families are helped to tackle issues early on, this reduces the likelihood of problems increasing and leading to more deep-seated difficulties in later life.”