The helpline charity revealed it received 453 calls and emails between July and September in 2015 from adults concerned about youngsters being left unattended - and children themselves.
Of these, 366 - more than 80 per cent - were so serious they were passed to police or social services.
The helpline received 1,729 calls and emails over the whole of 2015 from adults concerned about children being left to fend for themselves.
More than half of those referred to police or social services under the age of 10.
The NSPCC's ChildLine service also delivered 273 counselling sessions to children and young people last year who were worried about being left home alone.
Among the reports from worried children, one 12-year-old boy said: "I hate being home alone. I'm so bored and hungry.
"It's the school holidays and my dad is at work. He said childcare is too expensive and he can't afford it.
"Thing is I don't really know how to make anything to eat apart from make myself some toast but I'm sick of eating that every day.
"I thought about calling some friends over but I would be embarrassed because the house is so dirty. It makes me really sad."
A 14-year-old girl told the charity: "I feel so lonely. I'm at home alone and am not feeling well but there isn't anyone here that can look after me.
"I've been in bed all day and just feel like crying.
"Both my parents work really hard and I do feel sorry for them but it's the school holidays and all my friends have been visiting other countries while I have to stay at home.
"Everyday I'm home alone for hours until they both get back from work and it's going to be like this until school reopens in September. I feel so down."
The law does not give a minimum age for leaving children alone at home but it is against the law if it puts them at risk.
Sharon Copsey, from NSPCC South West England said: "Summer holidays can be a fun time for children but it's also when they are more likely to be left home alone as parents face increasing childcare pressures. This could explain why we see a spike in calls to our helpline during these months.
"Leaving your child home alone can be a difficult decision as children mature at different ages - there is no 'one size fits all' answer. But it could put them at greater risk of accident or injury.
"So I would urge parents to use their common sense when deciding if their child could cope. They should also ask them how they feel about being left alone and talk to them about what to do in an emergency. Parents are best placed to know what is right for their child so it vital there is flexibility for them to decide."