Steve Fouch, from the Christian Medical Fellowship (CMF) said nurses feel like "they're being asked to make bricks without straw" in terms of their workload, and a "loss of good will and a loss of moral" across the whole of the NHS because of pay and working conditions.
He was speaking after David Cameron made a speech in Birmingham, reiterating a pledge to make all NHS services in England, including GP surgeries, open seven days a week.
The pledge prompted the Royal College of Nursing to threaten strike action, if the move is funded by cutting the extra pay they get for working unsociable hours like weekends and holidays.
The King's Fund, an NHS thinktank, has said David Cameron's pledge to give the NHS an extra £8bn a year is not enough to fund opening all of its services seven days a week.
Steve Fouch, a nursing expert from the Christian Medical Fellowship, told Premier's News Hour: "I think there's a loss of goodwill and a loss of moral in the NHS at the moment.
"Where are the staff going to come from? We're facing staffing shortages in nursing and midwifery at the moment across the NHS.
"We've got a big problem with general practice; there are something like thirty percent of general practice training placements were left unfilled last year.
"How are you going to pay for all of this? There was a report earlier this year that said the NHS needed an extra thirty billion pounds funding a year by the end of this decade just to keep running as it is.
"There's a genuine anxiety because nurses' pay has been frozen for... several years... there's a genuine feeling amongst many in the nursing profession that they're being asked to make bricks without straw."
Listen to Premier's Aaron James speak to Steve Fouch on the News Hour.