A joint inspection by Ofsted and the Care Quality Commission found a raft of failures in SEND support across the Wiltshire town, which have left parents and carers "upset, angry and concerned", according to a letter sent to health and council chiefs.
Concerns include a failure to meet the statutory deadline of ensuring all SEN statements are converted to education, health and care plans, which are necessary to effectively plan support.
Where plans are in place, health and council leaders are failing to ensure they are of a high quality or are offering children the right support needed to improve their life chances.
"Parents and carers who contributed to the inspection are overwhelmingly negative about their dealings with the local area," said the inspectorates' letter.
"They told inspectors that they consider the future for their children as ‘bleak'."
The letter, which follows their visit in November 2018, adds: "Many young people with SEND who spoke to the inspectors are frustrated with the local area. They informed inspectors that they are not listened to by officers and ‘have been failed' with the quality of provision they receive."
In addition, care services are "weak" and are not providing "the necessary support, care and guidance for children and young people with SEND".
Inspectors also found that joint commissioning of support is "underdeveloped" and health and council leaders have been too slow to tackle weaknesses in SEND support.
Attainment for pupils with SEND "are not good enough", inspectors also warn.
"For example, those in mainstream secondary schools and young people aged 16 to 25 with SEND, do not achieve the outcomes of which they are capable," adds their letter.
Concerns are also raised about the higher than average proportion of fixed-term exclusions among pupils with SEND, particularly in secondary schools.
However, inspectors do acknowledge that a self-assessment by health and council chiefs of SEND services also highlighted weaknesses and work is under way to improve the quality of support.
A statement from Swindon Borough Council and the town's clinical commissioning group (CCG) says that action being taken includes working with the Department for Education's behaviour tsar Tom Bennett to help schools with issues around behaviour and exclusions.
"The inspection recognised that our self-assessment was accurate and there is much yet to do," said Mary Martin, the council's cabinet member for children and school attainment.
"We are heartened that the inspectors recognised that a lot of work is already well under way to ensure that the local area meets the disability and special educational needs reforms.
"Overall we very much need to up the pace of our improvement programme so we improve the outcomes for children and young people more quickly."
Both the council and CCG have been asked by inspectors to submit a written statement of how they intend to improve the quality of SEND services.
Meanwhile, a joint CCG and Ofsted inspection team has also criticised the quality of SEND support in Staffordshire, following a visit last November.
"Ineffective leadership has resulted in a fragmented and dysfunctional approach to education, health and care agencies working together," it states.
"This means that families do not get the help and support they need for their child."
Inspectors were told that there is a sense of diminishing inclusivity in Staffordshire and that children and young people are not often placed in the right educational establishment.
The poor academic achievements of pupils with SEND is another concern, according to inspectors, who have ordered local council and health chiefs to submit a plan of action on how they intend to improve support.
"We are aware some areas need improving and we are clear that we must do more with our partners to ensure families receive the right support at the right time," said Mark Sutton, Staffordshire County Council's cabinet member for children and young people.