The amendments will compromise GPs ethically and increase their workload, experts say.
The Mental Health Bill, which was first announced in 2001, was heavily criticised by mental health professionals and was condemned by a parliamentary committee in 2005.
Its plans for additional powers to detain the mentally ill caused the most concern, and it is these are still set to be introduced as modifications to the 1983 Mental Health Act.
Patients cannot currently be detained for treatment in hospital unless their condition is considered 'treatable'. This would no longer be the case under the proposed law, which would allow patients with untreatable disorders to be detained if they posed a risk to themselves or others, and clinicians believe that 'appropriate treatment', such as cognitive behavioural therapy, might alleviate their symptoms.
'The hurdle of treatability has been too high,' mental health czar Professor Louis Appleby said. 'Clinicians are operating in a grey area and can't admit patients in genuine distress, with suicidal thoughts or known personality disorder.'
The planned Bill would also have given community mental health teams the power to force treatment on patients released from detention. This measure is intended to reduce the number of people who relapse once released.
Other changes would have allowed social workers to replace psychiatrists as the professional in charge of some patients' care.
Dr Alan Cohen, south London GP and director of primary care at the Sainsbury Centre for mental health, said GPs would be affected if these changes were still implemented.
'How are GPs to know someone is receiving compulsory treatment, for example during an out-of-hours visit?' he said. 'I doubt community mental health teams will be available 24 hours per day seven days a week, so this is where those receiving compulsory treatment will turn up.'
Dr Cohen said it was vital primary care workers were involved in the implementation of the new laws.
However, Dr Chris Manning, chief executive of Primary Care Mental Health and Education, said GPs should fight such plans.
'We still need to fight this all the way,' he said. 'I would prefer to see legislation like in Scotland, where the bill is based on civil rights, not risk management.
'GPs will be very unhappy to treat people receiving compulsory treatment because it's ethically unsound. And patients will be deterred from seeking help, including with GPs, which is already a big problem in certain ethnic groups.
'Every professional and patient group interested in mental health is against this and I just hope we can beat it.'