A new 24-hour health hotline is failing patients, according to an undercover television investigation.
Channel 4's Dispatches programme, NHS Undercover, found the non-emergency 111 system had staff shortages, long waits for callers and in some cases ambulances were being called out unnecessarily.
Two reporters posing as trainee call handlers secretly filmed at centres run by the private health care company, Harmoni, which has a third of the hotline contracts in England.
Dispatches said the footage shot at the branches in Dorking in Surrey and Bristol reveals "serious failings in the system".
The programme said its reporters found many patients were left waiting for longer than the 10-minute target for a callback from a clinician and some workers who were not clinicians were filmed giving medical advice to the undercover journalists.
They also uncovered concerns about training, staff shortages and fears the system is leading to unnecessary callouts for ambulances, putting additional pressure on an already stretched A&E service, Dispatches said.
One Harmoni call centre manager was secretly filmed admitting: "We had a very bad service. Still realistically on the weekends we still are unsafe. We don't have the staff to deal with the calls that are coming in."
Peter Carter, chief executive of the Royal College of Nursing, called for "immediate intervention" at "governmental level".
However, a spokesman for Harmoni told the programme: "We provide a clinically safe service.
"We expect all staff to only provide advice according to their role and their level of training and take a zero-tolerance approach to any breach. Our audits show no evidence of widespread poor practice."
The 111 phone number was rolled out in April, replacing NHS Direct as the number to call for urgent but non-emergency care.
But the line suffered many teething problems, with patients complaining of calls going unanswered, poor advice given, and calls being diverted to the wrong part of the country.
Harmoni added: "Our staffing levels are extremely robust with around one clinical advisor to every four health advisers.
"Staffing levels are agreed and shared with local commissioners and NHS England and are reviewed weekly.
"While it has been acknowledged that NHS111 services generally did not get off to a good start, we and other providers have successfully delivered substantial improvements including recruiting 180 more advisers.
"Most providers found the pace and timing of the rollout of a new national service receiving a mix of calls created initial challenges."
Harmoni said it would "fully investigate" any specific concerns raised in the programme.
A spokesman for NHS England said: "We know there were problems with performance in some areas over the Easter period this year.
"These problems were mainly due to the local providers of 111 services having insufficient call handling capacity in place. NHS England has been clear with providers and commissioners that this level of performance is unacceptable.
"Some patients had to wait too long for calls to be answered. This has now been resolved and calls are consistently answered within 60 seconds. In addition to this, problems with the transfer of calls to clinicians have also largely been resolved."
"NHS England's role is in supporting local GP-led CCGs as commissioners of services. It is our absolute priority to ensure patients get a high quality, safe service wherever they live and where providers are failing to meet standards, NHS England is working with local CCGs who took over the commissioning of the NHS 111 service on 1 April 2013 to ensure these providers have plans in place to improve and meet the required standards.
"There is no evidence that NHS 111 is increasing demand on A&E or 999.
"We have no evidence to suggest there are not enough clinicians in NHS 111 call centres, although this is something NHS England will be looking at as part of its ongoing work monitoring the service."
NHS Undercover: Channel 4 Dispatches will be broadcast tonight at 8pm.
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