The Mental Health Alliance will meet later this month to decide its future. The coalition of 77 charities, service user groups and professional bodies has been campaigning for a better bill since 1999.
"We are never ever going to be in that situation again when a government can come along and say 'we're imposing this piece of mental health legislation'," said Paul Corry, public affairs director at Rethink.
Corry said the alliance might not continue in its current form, but loose coalitions may continue to campaign on issues such as anti-discrimination.
Young Minds' policy and innovation manager Kathryn Pugh said she hoped the alliance would stay to scrutinise the code of practice on the new act. "Even if the formal alliance goes, the working relationships and benefits of partnership working and sharing expertise will stay with us for a long time."
Mind chief executive Paul Farmer said the alliance had helped build a consensus across the mental health sector that "wasn't there" five years ago.
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