A study by the Council for Disabled Children and The True Colours Trust, found there has been a 50 per cent increase in the number of disabled children with complex needs since 2004, from 49,300 to 73,000.
A key factor in the increase is the fact that medical breakthroughs have increased the life expectancy of babies born with serious disabilities.
The sharpest rise has been among the number of children with severe forms of autism, with this group of children doubling in numbers since 2004, to 57,615.
However the report found that vulnerable groups of children are struggling to get the support they need.
It says that the proportion of children with a disability supported by local authority children's services departments has fallen in each of the last five years, and now stands at 0.4 per cent of all 0- to 17-year-olds.
"This suggests that thresholds for assessing disabled children's services have been rising," the report states.
It also details how 41,500 children and young people with a learning disability or autism are on waiting lists to see a mental health specialist.
The report suggests the true number of children with complex needs may be even greater, as many children with multiple health and education needs are in independent special schools, which are not required to return school census data to the Department for Education.
The lack of clear data is a particular concern for Council for Disabled Children director Christine Lenehan.
"You'd think that because these disabled children are known to health services, social services and education teams, we'd have a good idea of the numbers involved," she said.
"That simply isn't the case. The national data on disabled children is not fit for purpose: it has gaps, anomalies and inconsistencies, and raises the question how can we plan to meet the needs of these children and their families, when we don't know what those needs are?"
The report calls on the Department of Health to put in place incentives and penalties around data sharing regarding disabled children with complex needs.
Meanwhile, the DfE is being urged to require independent schools to provide school census pupil data and include a special focus in its data bulletins on those with complex needs.
It also calls for Ofsted local area inspections to better monitor how well data on disabled children and young people is being used in commissioning.
