Almost one in five children starts school not able to communicate at the expected level, the Children’s Commissioner for England has said.
And many experience communication problems throughout their childhood, often facing a postcode lottery of care, Anne Longfield added.
Need to know: Speech, language and communication needs - a quick guide
Ms Longfield pointed to an "enormous variation" in how much is spent per child, depending on where they live.
And she said a lack of joined-up services and overall responsibility meant that children with speech and language difficulties could fall through the gaps, wait months to be seen or may not be seen at all.
In 2018-19, the top 25 per cent of areas spent £16.35 or more per child on speech and language therapy, compared with 58p or less per child in the bottom 25 per cent of areas.
In her report, Ms Longfield said: "Additionally, while overall spending has increased (albeit by only 2 per cent per child), it has fallen in many areas.
"More than half of areas experienced a real-terms decline in spending per child."
Figures show that 11 per cent of two-year-olds are below the expected level of communication, while 18 per cent of five-year-olds – 114,822 children – are also not reaching their expected level.
Overall, 193,971 primary-school children – about 4 per cent of the total – are on the special educational needs register for speech, language and communication needs, although the true level of need is thought to be higher.
The problem is also worse among pupils from deprived backgrounds, with 23 per cent of those five-year-olds who are eligible for free school meals failing to meet expected levels. By contrast, 13 per cent of pupils who do not receive free school meals do not reach expected levels.
Research shows that children with poor vocabulary are twice as likely to be unemployed when they grow up. And more than 60 per cent of children in young-offender institutions have communication difficulties.
In her new report, Ms Longfield states that no single body exists that can be held to account for spending on children's speech and language services.
Her office found that, among children with an identified speech and language need, the top 25 per cent of local authorities spent at least £291.65 per child. But the bottom 25 per cent of local authorities spent £30.94 or less.
Ms Longfield said: "Communication skills are vital for children starting school and for improving social mobility throughout a child's education.
"We should be very concerned that almost one in five children aged five is behind in speech and language development – and yet more than half of areas in England have seen a real-terms fall in spending on speech and language therapy in recent years.
"The next prime minister must make school readiness a priority if we are to give all children the chance to thrive. A well-resourced strategy for addressing speech, language and communication needs must be part of that."
Ms Longfield made a number of recommendations in her report, including ensuring local areas are held to account and that expenditure data is collected on an ongoing basis.
She said that all areas should have a plan looking at need in their region and how it should be met, with particular regard to disadvantaged children.
Kamini Gadhok, chief executive of the Royal College of Speech and Language Therapists, said the current situation was "unacceptable", adding that the college’s own survey had shown that 59 per cent of parents already have to fight to get the support their children need.
And Anntoinette Bramble, of the Local Government Association, said: “Significant financial pressures on councils’ public health and special needs budgets is severely impacting on their ability to support children with early language development.
“Councils are also working closely with local early-education and childcare providers and clinical commissioning groups to make sure children are ready to start school, but insufficient funding is impacting on the quality of provision and support for children with special needs, as providers struggle to balance budgets.”