More than 100 MPs have signed an early day motion, which was tabled by Labour MP Lynne Jones last month, condemning the Mental Health Bill.
Forty-six of the MPs are Labour, enough to overturn the government's 65-strong majority if all signatures were turned into votes. The opposition parties have already come out against the bill.
Jones said: "I am concerned that the proposals do not address the needs of people who may suffer from mental ill-health and consequently will not be effective in safeguarding the public in the rare cases where there is a danger."