The Liberal Democrats are calling for an end to the "postcode lottery" on special educational needs and disabilities (SEND).
There are huge differences in support offered in schools, linked to extra costs, says party leader Sir Ed Davey.
He wants a new national champion to coordinate support for children with the most complex needs as part of a £300m boost.
The government published its own SEND improvement plan last year.
Parents must get an Education, Health and Care (EHC) plan for their child to access tailored SEND support.
However, the latest government statistics show just over half the plans were not provided within the legal 20-week deadline, leaving tens of thousands of children without support.
Lib Dem analysis shows stark differences between councils across the country, with 96% of requests for families in Leicestershire breaching the time limit, whereas nearly every family in Calderdale got their plans in time, with only 1% of requests missing the deadline.
The party believes part of the problem with the delays is down to financial barriers for schools, as each establishment must pay £6,000 per child towards additional SEND support.
Sir Ed, who is a carer for his disabled son, announced the new policy on a local election campaign visit to Brentwood and Chelmsford in Essex, where he visited a special educational needs charity and spoke to families.
He outlined how Lib Dems would set up a dedicated national body for SEND, to act as a champion for children with complex needs and ensure they receive tailored support.
Sir Ed wants to see an extra £300m dedicated to SEND and promised to set out fully costed plans in his party's manifesto ahead of the general election later this year.
Speaking on the visit, he said: "When a child or young person has particular needs that cost a lot of money to support, we're arguing that should fall under the national budget, and that would relieve the pressure on councils...
"That would make sure that families and local authorities are working together and not against each other."
'Urgent action'
Investing now would cut bills in the long-run, Sir Ed suggested, as identifying SEND early on can prevent more complex needs developing.
"Urgent action is needed to ensure that all children can access the tailored learning and support they need," he added.
A Department for Education spokesperson outlined how the government is "reforming the system across the country, with earlier intervention, consistent high standards and less bureaucracy".
"We want all children to have the chance to reach their potential, which is why we are increasing funding for young people with complex needs by over £10.5bn next year - up 60% in the last five years," they said.
That included investing more than £21m, which is being used to train 400 more educational psychologists and to increase the number of teaching assistants by nearly 60,000 from 2011.
From BBC