Charities have accused the government of having "little regard" for mental health and a "major breach of trust" after key reforms did not appear in the King's Speech.
The King's Speech, which set out Rishi Sunak's legislative agenda at the state opening of parliament, did not contain any reference to the Mental Health Bill, which featured in the Conservatives' 2017 and 2019 manifestos.
The bill contained provisions to reform the Mental Health Act 1983, which currently allows people with autism and other learning difficulties to be detained and placed in mental health hospitals.
Charities also supported the bill on the grounds it was intended to address concerns that black people are four times more likely to be detained under the Act.
But when the monarch delivered the King's Speech - the first in over 70 years - there was no reference to the bill, in a move one charity described as "incomprehensible".
Former justice secretary Robert Buckland also told Sky News he was "extremely disappointed" not to see the bill in the speech, while Rosena Allin-Khan, Labour's former shadow minister for mental health, accused the Tories of "dither, delay and failure".
Dr Sarah Hughes, chief executive of Mind, said the Mental Health Bill provided "a crucial chance to prevent people being stripped of their dignity, voice and independence when they are sectioned".
"That chance has now been missed," she added.
She accused the government of having "broken its promise to thousands of people, their loved ones and the nation as a whole to reform the Act".
"This is further evidence of how little regard the current UK government has for mental health," she added.
The blueprint for the Mental Health Bill was published by the government in June 2022 and outlined "once-in-a-generation" reforms to treat "mental health with the same urgency as physical health".
It was scrutinised by a joint committee earlier this year but there has been no formal response from the government.
Dr Hughes said it was "incomprehensible" that the legislation was "de-prioritised" when more than 50,000 people were held under the Mental Health Act last year.
"There could not be a worse time to abandon this bill, especially given the recent string of exposés revealing unsafe mental health care across the country," she said.
Centre for Mental Health chief executive Andy Bell said the decision to drop the bill in this parliament "represents a major breach of trust to the many people who have experienced the Act first-hand and campaigned for essential reforms".
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He said the current Mental Health Act was "outdated", "discriminatory" and gives "too little regard to people's rights and dignity".
"A modernised Act could ensure people have better safeguards for their rights and dignity and more of a say in how they are treated," he added.
A spokesperson for the Department of Health and Social Care said it was going "further and faster to transform our country's mental health services, with up to an additional £2.3bn being invested annually by 2024 to expand services, so an extra two million people can get the support they need".
"We are continuing to pilot models of Culturally Appropriate Advocacy, providing tailored support to hundreds of people from ethnic minorities to better understand their rights when they are detained under the Mental Health Act.
"Anyone receiving treatment in an inpatient mental health facility deserves to receive safe, high-quality care and to be looked after with dignity and respect and we are committed to ensuring this happens."