The timetable for implementing reforms of the NHS is "ambitious" but not unachievable, the Health Secretary will say today.
Andrew Lansley has come under fire over the speed of his proposals, which will see GPs take control of the NHS budget from 2013.
NHS trusts, which manage the cash at the moment, will be abolished.
Today, Mr Lansley will seek to reassure doctors attending a Royal College of GPs (RCGP) conference in Harrogate, promising full support for their new role.
He will accuse the previous Labour government of having "dithered and delayed in its approach to NHS reform".
Labour's approach "fell apart entirely" in the last three years due to a lack of clarity about what ministers were trying to achieve, he is expected to say.
"As a Government, we will not repeat these mistakes.
"That is why I have been so clear about setting out our ambitions for the NHS, the reforms we will make to achieve these ambitions, and by when we will make the reforms.
"It is true that the timetable is ambitious, but it is not a timetable which is unachievable.
"With two-and-a-half years with which to learn from pathfinder commissioning consortia and establish shadow arrangements, there is ample time for practices that do not yet feel ready, to secure capability collectively.
"These reforms give GPs the overall responsibility for the design of services, which meet their patients' needs, and facilitate a quick response when failures in those services arise."
He will tell GPs he does not intend to burden them with paperwork or involve them in the "minutiae of administration".
Support will be available from health trusts, local authorities or "external partners".
Mr Lansley will say there is almost three years to "consult, dry-run and put reforms into practice on the ground" before GPs take on responsibility.
A survey published last night by the BBC showed most GPs are sceptical the overhaul will actually benefit patients.
The poll of 827 doctors found fewer than one in four think putting GPs in charge of the health service budget will lead to improvements.
Just 23 per cent said the reforms would benefit patients, with 45 per cent saying they would not and 32 per cent expressing no opinion.
Meanwhile, just 25 per cent of doctors said they would be willing to take on the extra responsibility of planning and buying in services, with 57 per cent saying they would not do it and 18 per cent expressing no opinion.
GPs also expressed doubts about becoming so closely involved in commissioning in specialist areas such as cancer and paediatrics.
On Monday, the RCGP questioned the speed and cost of the plans and warned of a possible "erosion of the crucial relationship" between doctors and patients.
It also pointed to "grave" concerns about the use of private companies to run NHS services and the "loss" of expertise in existing NHS trusts.
The Government's White Paper has further sparked concern at the Royal College of Nursing (RCN) and the British Medical Association (BMA).
While the RCN supports the underlying principles of giving health workers greater freedoms, it has said the proposals are "untested".
The BMA said it was not against the whole vision, but had concerns that the changes could affect the service's "stability and future".
And Unison has warned of possible instability, saying this could have an impact on patients.
The BBC survey, carried out online between September 23 and 30, found most GPs do not believe they are well prepared to take charge of commissioning in several key areas. These include cancer, emergency hospital care, mental health and paediatrics.
Seven out of 10 also said the planned changes would lead to the private sector taking on a bigger role in the NHS.
Speaking to BBC Radio 4's Today programme ahead of his speech, Mr Lansley said he was not rushing the reforms.
"What I'm talking about is a very substantial change, of course it is, but over the course of two and half years and in circumstances where there will be two full financial years, during which first the general practices in consortia will be able to establish how they see it working, and then to actually to see it work during the course of a whole year."