The body representing doctors in Britain has called for a delay in the launch of a new non-emergency helpline, saying it is putting patient safety at risk.
The British Medical Association (BMA) has written to Sir David Nicholson, chief executive of the NHS, to highlight its concerns about the 111 advice line service, which is due to go live in England on Monday.
Dr Laurence Buckman, chairman of the BMA's GP committee, questioned the quality of advice given out and said patients had been forced to wait for hours for advice, the BBC said.
He also said the "chaotic mess" of the 111 service was straining parts of the NHS that were already stretched, potentially putting patients at risk.
Dr Buckman said: "There have been widespread reports of patients being unable to get through to an operator or waiting hours before getting a call back with the health information they have requested. In some areas, such as Greater Manchester, NHS 111 effectively crashed because it was unable to cope with the number of calls it was receiving. The quality of advice being given out has also been questionable in some instances."
He added: "The BMA has been warning the Government about the problems with NHS 111 for almost two years. They must finally act to ensure that patient safety is guaranteed."
Calling for a delay of the launch until it is "fully safe for the public", he said: "We cannot sacrifice patient safety in order to meet a political deadline for the launch of a service that doesn't work properly."
Ministers admitted on Tuesday the introduction of the 111 telephone service had "teething problems". Health minister Lord Howe said he was aware of problems in two particular areas, but said pilots of the new service showed a high rate of patient satisfaction.
NHS England - formerly the NHS Commissioning Board - said it was aware of difficulties in introducing the new number in some areas, but that it was "confident that measures now in place will ensure resolution of these early problems".
The new 111 service is aimed at helping people who urgently need medical help or advice but are not facing a life-threatening 999 emergency.