The RCN said the Bill posed a ‘serious threat to the NHS’ and did not deliver on the original aims of the reforms.
It said the NHS will face a ‘very bleak future’ as a result of the Health Bill, the rising financial pressures and the public health challenge.
RCGP chairwoman Dr Clare Gerada said the College will also call for the withdrawal of the Bill 'should the situation warrant it' after a poll earlier this month found 98% of RCGP members think it should call for the Bill's withdrawal.
Dr Gerada said: 'I have written again to the health secretary and given him another opportunity to meet with us, inviting him to suggest ways in which we can move forward, although I still await his official response.'
The BMA also said it was pleased the RCN had toughened its stance on the Health Bill and promised to work with the nursing union to ensure the 'flawed Bill' was withdrawn.
RCN chief executive Dr Peter Carter said the decision to oppose the Bill was taken following the government’s failure to listen to the concerns of nurses and clinicians.
He said: ‘The RCN has been on record as saying that withdrawing the Bill would create confusion and turmoil, however, on the ground, we believe that the turmoil of proceeding with these reforms is now greater than the turmoil of stopping them.
‘The sheer scale of members' concerns, which have been building over recent weeks, has led us to conclude that the consequences of the Bill may be entirely different from the principles which were originally set out. The RCN feels that these concerns are so fundamental that we must now oppose the Bill.’
The DH said it was ‘disappointed’ that the RCN had toughened its stance on the Health Bill.
A spokeswoman said: ‘During the course of the past twelve months we have been working with nursing groups to shape our plans for a modern NHS. For example, nurses will be represented on local clinical commissioning groups.
‘The RCN has conflated the Health and Social Care Bill with issues about the need for the NHS to spend its money more efficiently. The Bill is needed to empower doctors, nurses, and other frontline healthcare workers across the NHS to take charge of improving care.’