Doctors' leaders have written to the Government over “serious misgivings” about the rollout of a new number for non-emergency services.
The British Medical Association (BMA) said the deadline for introducing the new 111 telephone number needs to be relaxed from its current date of April 2013.
In the short-term, 111 will run alongside existing local telephone services and NHS Direct, but it will eventually become the single number for non-emergency care.
In a letter to Health Secretary Andrew Lansley, the BMA said its doctors had raised a "number of serious problems and concerns" in the pilot schemes used to test the number.
These issues, together with not allowing enough time for doctors to get involved in the project, mean there could be an increased burden on A&E and GP surgeries from patients not getting the help they need, it said.
Concerns have also been raised about the level of medical training staff will have in the new service, and whether they will be able to direct patients to the right help.
Dr Laurence Buckman, chairman of the BMA's GPs committee, said: "GPs have been telling us for quite some time about problems with the way the NHS 111 is being rolled out and the wider impact it could have on the health service.
"For example, in Shropshire GPs are worried that patients will actually receive lower quality care as the clinicians who triage all calls to their out-of-hours provider are to be replaced by non-clinicians when NHS 111 takes over.
"If there was a more flexible deadline in place then local commissioners would have time to work out a solution with NHS 111 so that this option could be kept for their area.
"The results of the pilots are due to be published imminently and we are worried that the strict deadline in place at the moment means lessons from these won't be learned and mistakes will just be repeated.
"If there isn't a pause then the Government could end up implementing something which doesn't work to the benefit of all patients, which could unnecessarily overburden the ambulance service and GP surgeries, reduce the quality of existing out-of-hours services and ultimately cost the taxpayer a lot of money.
"More flexibility would allow clinical commissioners to get properly involved in how NHS 111 is being rolled out in their area, making sure it is sensitive to local need.
"GPs are happy to work with NHS 111 to iron out any problems, but they need time in order to do that."
The 111 number is currently being piloted in County Durham and Darlington, Luton and the East Midlands.
Last month, the Royal College of Nursing (RCN) warned that up to 300 frontline staff at NHS Direct could lose their jobs under new shift arrangements as the new 111 number is phased in.
Workers who are unable to join the new shift pattern will lose their jobs, including those with flexible working arrangements already in place, such as those caring for children, it said.
NHS Direct denied the claims, saying 79% of frontline staff had been offered "one of their preferred roster patterns" and there were enough spaces for everyone to be allocated to a new roster.
NHS Direct is not the only organisation tendering to deliver NHS 111 services.
The new system could allow private and GP out-of-hours providers and the ambulance service, among others, to deliver the service.
Ambulance Service Network director Jo Webber said: "Ambulance services agree with the British Medical Association that the roll-out of NHS 111 across the country should only take place once the pilot schemes have been evaluated properly.
"The timeframes for putting NHS 111 in place are currently just too tight to be sure it will work in a consistent way across the country.
"NHS 111 has massive potential to improve care for patients as it can put them in touch with the most appropriate service quickly.
"But it will only realise this potential if providers of services and those who commission them come together in a coordinated and strategic way to make it work.
"A headlong rush to implementation will not necessarily benefit patients."
Public Health Minister Anne Milton said: "The BMA supports the principles of the NHS 111 service - it will benefit patients by improving access to NHS services and ensuring they get the right care at the right time.
"We will consider the BMA's concerns.
"We agree that any long-term decision should be made with full approval from local commissioning groups.
"They should be fully engaged with the approach to delivering NHS 111."
Nick Chapman, NHS Direct chief executive, said: "NHS 111 has the potential to make it easier for patients to access advice. However the results of the pilots need to be available before Clinical Commissioners commit to long term contracts to deliver NHS 111 in their area. It would be extremely worrying if the opportunity to learn from the pilots, and make appropriate changes, was lost because of unrealistic rollout deadlines.
"Patients really value being able to get advice they trust on the telephone and web. That may be swift onward referral to another health service or advice that allows them to look after themselves at home safely where this is appropriate.
"NHS Direct is leading on four of the seven pilots, and the preliminary results are showing us that the service refers more patients for face to face care than existing services. As a result of this, changes will probably be needed. This may include greater use of clinicians (nurses and doctors) to complete more in-depth clinical assessments on the telephone in order to avoid unnecessary referrals to GP surgeries, A&E and ambulance call outs."