British Medical Association chair James Johnson said: 'If primary care trusts have run out of money, they clearly cannot buy a lot more operations.
'The second stumbling block is one of workforce planning. For surgery to be performed over extended hours we have to have the skilled staff to do the work and there has been no adequate workforce planning to allow this to happen.'
Mr Johnson was responding to prime minister Tony Blair's attempt to kickstart the drive to meet next year's 18-week target by announcing that hospitals should perform operations in the evening and at weekends.
Mr Blair visited King's College Hospital foundation trust in south London to promise that the 18-week target would 'transform' the NHS.
'The NHS cannot carry on as if the world was still as it was decades ago,' he said.
King's Fund chief executive Niall Dickson said he was 'cautiously optimistic' that the 18-week maximum wait target will be hit, but he said: 'This will be a much tougher challenge than the government, or indeed any of us, initially anticipated. Just over a third of patients are currently being treated within 18 weeks, while the average waiting is closer to 30 weeks.'