Research commissioned by the children's commissioner for England also found that 57 per cent of areas reported a real terms cut in spending on speech and language therapy (SLT) in the year up to March 2019, with just 23 per cent of areas seeing a real-terms increase in spend per child.
The We Need To Talk: Access to Speech and Language Therapy report states that councils and clinical commissioning groups (CCGs) spent £166m on SLT services in 2018/19, equating to £10.12 per child.
It analysed the amount spent by all agencies on SLT services on a regional basis, finding substantial variations between English regions.
The top 25 per cent of regions spent at least £16.35 per child across the entire 0-17 population, while the bottom 25 per cent spent 58p or less.
There was also significant variation when assessing level of spending on services for "children with recognised SLT needs".
The top 25 per cent of councils spent at least £291.65 per child, with the bottom 25 per cent spending £30.94 or less.
When looking at spending across the entire child population, London had the highest spend (£7.29 per child), followed by the South East (£5.73) and East of England (£4.83). It was lowest in the East Midlands (£0.34), West Midlands (£0.90) and Yorkshire and Humber (£1.18).
Among CCGs, the North of England had the highest spend per child (£17.61) followed by London (£17.14). The lowest was in the Midlands & East NHS region (£10.20) followed by the South of England (£13.54).
The report also found that only half of CCGs and local authorities in England are jointly commissioning services, even though they are expected to do so for children with identified special educational needs. This introduces the risk that local areas are not joining up all the information they hold and are unable to ensure that they are providing services for all children in the area who need them, states the report.
It was found that CCGs spend substantially more on SLT services than local authorities. CCGs were responsible for 69 per cent of spend, compared with 25 per cent for local authorities. Other agencies made up the remaining six per cent.
Longfield said she was "very concerned" that almost one in five children aged five is behind in speech and language development.
"Yet more than half of areas in England have seen a real-terms fall in spending on speech and language therapy in recent years," she added.
Among the recommendations in the report is a call for the government to require that all local areas have a strategic plan in place that assesses the level of children's speech and language need in their area, giving particular consideration to disadvantaged children.
Longfield also called on the next Prime Minister to make school readiness a priority.
"A well-resourced strategy for addressing speech, language and communication needs must be part of that," she said.