Doctors' leaders say they have "serious concerns" about the rollout of the NHS 111 phoneline in England.
The free one-stop number for patients with urgent, but not life-threatening symptoms, is planned for April 2013.
The British Medical Association says existing services may come under strain and wrote to the health secretary asking for the deadline to be relaxed. Other organisations echo its worries.
The government said it would consider the issues raised by the BMA.
Last month the new non-emergency 101 police phone number was launched nationwide and has already received more than 2.5 million calls.
The 101 number is intended for calls that do not require an emergency response from either the police or fire service.
'Lower quality care'
When a patient calls 111, a trained operator - who is trained in the same way as a 999 operator - can send out an ambulance, if that is what's needed, put someone straight through to a nurse, book an out-of-hours GP appointment, or direct the caller to a pharmacist or dentist.
In contrast, the existing NHS Direct service is also initially answered by trained non-clinical staff, but they do not have the capacity to request ambulances or book appointments - and patients receive a separate call back if they need to speak to a nurse or doctor.
NHS 111 is currently operating in Luton, County Durham and Darlington, Lincolnshire, Nottingham City, the Isle of Wight and parts of Derbyshire and Lancashire.
The BMA says it supports the principle behind NHS 111, but that doctors in areas where the phoneline is being piloted had faced "a number of serious problems and concerns".
It also warns that areas where the service has not been piloted are setting up NHS 111 apparently without reference to lessons from pilot areas, and that the new clinical commissioning groups who will be responsible for local care are not being included in decisions about the new service.
Dr Laurence Buckman, chairman of the BMA's GPs committee, said: "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."
He said a more flexible deadline would give time for local doctors to work out a way to work alongside NHS 111.
Dr Buckman added: "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."
In his letter to Health Secretary Andrew Lansley, Dr Buckman said he wanted to express "serious misgivings" about the rolling out of the new NHS 111 service.
He also raised concerns about how the phoneline would fit in with existing services and provide value for money.
Nick Chapman, chief executive of NHS Direct, which is leading work in four of the seven 111 pilot sites, said the new service did have potential.
But he added: "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."
Health union Unison, ambulance services and nurses have also echoed the BMA's concerns about the plans.
Tom Sandford, director of the Royal College of Nursing in England, said: "Jumping the gun and introducing the service too early could create a fragmented, ineffective service where patients may not receive the level of care they want and need from a telephone triage service.
"This in turn could lead to increased numbers of anxious patients visiting GP surgeries and A&E departments, adding further pressure to the already over-stretched teams."
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."