And about 130,000 carers aged five to 17 may be unknown to local authorities, according to the Children’s Commissioner for England.

Anne Longfield said it was ‘absolutely unacceptable’ that so many young carers were going under the radar and she accused town halls of failing them.

She also warned that this is likely to be the tip of the iceberg because councils did not count children who looked after family, friends and neighbours who were hooked on alcohol and drugs.

For the first time, the commissioner has collected data to find out how children who look after sick and disabled relatives are themselves supported.

The 2011 census identified 166,363 children providing unpaid care. Based on a survey of 152 local authorities, she calculated 33,506 young carers received support, meaning 132,857 – or about 80 per cent – did not get any help.

Miss Longfield found that many councils prioritised their legal duty to assess referrals over actually providing support.

One service provider, speaking anonymously, said: ‘We are essentially carrying out assessments as a tick-box exercise. We are prioritising bureaucracy and not actually considering what we need to best help young carers.’

Miss Longfield said: ‘Caring for relatives often places heavy emotional and physical burdens on children. It can lead to them missing out on education and also opportunities to make and spend time with friends.

‘I am also concerned that some local authorities say that they are supporting young carers under the age of five. I will be following this up with those local authorities to clarify exactly what it is that these children are doing.’

She added: ‘It is absolutely unacceptable to have so many children with caring responsibilities going under the radar’.

The report found that of the 18,746 young carers who were brought to the attention of councils in 2015-16, a third had not received an assessment.

A report three years ago found that one in 12 young carers spent 15 hours a week looking after a parent or sibling, that one in 20 miss school and that they are 50 per cent more likely to have special needs or an illness.

Councillor Richard Watts, of the Local Government Association’s children and young people board, said councils were facing a £1.9 billion funding gap for children’s services by 2020.

He added: ‘Unfortunately, this means that councils are forced to make increasingly difficult decisions about the level of support they are able to provide to children and young people in their area’.

A Department for Education spokesman said: ‘Young carers are this country’s unsung heroes, selflessly providing support for the people they love, but their own needs can often be overlooked. That’s why we changed the law to make sure young carers are identified and supported as early as possible, and councils are responsible for delivering this. Next year we will be publishing a strategy looking at the issues affecting these children and what more can be done to give them the help they need.’