More than six out of 10 child health practitioners have said that hunger and poor nutrition is leading to a deterioration in kid's health.
A survey of school nurses and dentists found children are not growing or putting on weight at the expected rate and suffering tooth decay due to unhealthy diets.
Some 65% said children’s health had got worse as a result of hunger and poor nutrition in the past year, according to a poll of 313 health practitioners.
Almost a third (28%) said that children were experiencing more frequent or worse problems with their health.
More than half (53%) said kids were not growing or putting on weight as expected while 58% noted low energy levels compared to other children.
Free school meals for all primary kids in London, says capital Mayor Sadiq KhanSome 55% noted changes in children's behaviour such as being unusually irritable, slow or anxious and 51% reported an increase in the frequency of mental health problems.
And 78% of respondents said they had encountered children’s teeth decaying or being damaged at a higher rate than usual.
The overwhelming majority (94%) of respondents from the School and Public Health Nurses Association, and the British Dental Association, backed extending free school meals to all primary kids to ensure they don't go hungry.
It comes as activists, politicians and civil society leaders take part in a week of action to urge the Government to extend the scheme.
Kevin Courtney, Joint General Secretary of the National Education Union, said: “With 4.2 million children living in poverty, it is incumbent on the Government to take action.
"These findings demonstrate the impact of child poverty on children's health and paint a worrying picture for their futures.
“Front line healthcare staff such as school nurses, dentists and teachers play a huge role in supporting young people to live the best lives they can, but these findings show that their jobs are becoming increasingly difficult in the face of rising poverty.
He added: "We can no longer afford the costs of inaction and must take action to prevent the health of our children from deteriorating further.”
Sharon White, Chief Executive of the School and Public Health Nurses Association, said: “This should not be happening in the fifth richest county in the world.
"We trust that as part of this week of action, that our decision makers listen, do and act urgently to provide universal Free School Meals.
'I know free school meals are a lifeline because I depended on them'"Without this, the downward trajectory of our children’s health outcomes will continue long into adulthood. They need and deserve much better.”
Eddie Crouch, Chair of the British Dental Association, said: “Our kids are born into a toxic food environment, and dentists see the results every single day.
“Tooth decay is the number one reason for hospital admissions among young children, and bad diets are fuelling it.
“Free School Meals is a simple step that would put prevention to work in every school in this country.”
Find out more about the Free School Meals week of action here.