Serious child harm cases reported by councils in England rose by nearly 20% during the first year of the pandemic, including a 19% rise in child death notifications, according to latest official statistics.

There were 536 serious incident reports in England during 2020/21, up 87 (19%) from 449 in 2019/20, and an increase of 41% on the number of incidents recorded five years ago. Children aged under one accounted for 36% of notifications last year.

The Local Government Association (LGA) called the rise “harrowing and a huge cause for concern”. It said extra pressure experienced by families during the pandemic may have fuelled the increase and said abuse was more likely to have gone unseen “behind closed doors” during lockdowns.

Councils are required to notify the independent child safeguarding practice review panel of all serious incidents involving children, including death, sexual abuse, serious or life-threatening injury or permanent damage to health as a result of abuse or neglect.

The LGA said the increase demonstrated the urgent need for extra government funding in the autumn spending review. It said councils in England had spent an extra GBP1.1bn on children’s social care over the past two years but soaring demand for safeguarding services meant they still overspent budgets by GBP832m in 2019/20.

A Guardian investigation last week revealed soaring numbers of children suffering abuse, neglect and poverty, with growing numbers of young people taken into care, especially in some of England’s most deprived communities.

Under successive lockdowns, the number of incidents of domestic violence has increased, alongside rising levels of mental illness. At the same time, child safeguarding concerns often went undetected because of the closure of schools and some childcare settings.

Anntoinette Bramble, the chair of the LGA’s children and young people board, said: “Supporting and protecting vulnerable children is one of the most important roles played by councils that want to ensure all children are safe, loved and thrive, so this rise in serious incident notifications is particularly harrowing and a huge cause for concern.

“The pandemic has put extra pressure on families, particularly those living in difficult circumstances, which can fuel harmful acts of abuse or neglect on children. Councils have been working hard with their partners to identify this and provide the help children need, but it is vital that children’s social care services are funded to meet this need.”

Steve Crocker, the president of the Association of Directors of Children’s Services, said: “This latest data on serious incident notifications over the past year is concerning but not surprising to local authorities whose staff are on the frontline of child protection investigations. Sadly, the data illustrates the impact of the pandemic on the most vulnerable children and young people and their families.”

The children’s social care system is currently subject to an independent review headed by Josh MacAlister. An interim report from the review published in June called it a “30-year-old tower of Jenga held together by Sellotape” that needed major reform and investment.

A Department for Education spokesperson said: “We know that the pandemic may have exacerbated the challenges many vulnerable children, including those in care and their families, may have faced but the increase in serious incident notifications is of concern.

“That’s why we prioritised vulnerable children throughout the pandemic by keeping schools open to many of them. We are also investing significant funding to improve safeguarding for infants and adolescents, support councils with high demand for children’s social care, and to improve outcomes for vulnerable children.”