The Government must throw its weight behind plans that could lead to the closure of NHS hospitals and some services, a think tank has said.
The King's Fund said controversial sustainability and transformation plans (STPs), which aim to save cash but will lead to some cuts, offer the best hope of making progress in the NHS.
But Chris Ham, the think tank's chief executive, said the aim of cutting hospital beds, which many of the plans set out, is not realistic at a time when hospitals are already running at full capacity.
STPs have been created in 44 areas of England, setting out how the NHS will change services while also saving cash.
In several regions, the aim is to close or downgrade entire hospitals, while others suggest closing A&E departments or maternity, or merging some services.
All focus on a desire to stem the rise in hospital admissions by providing care closer to people's homes.
The new King's Fund report said there is currently an "uncertain" level of political backing for STPs, which were ordered to be drawn up by NHS England.
It argues politicians must face up to having difficult conversations with the public about which services should close.
Reconfiguring services "stands little chance of being implemented without support from the Government and a willingness to back NHS leaders where the case for change has been made", it adds, saying such support is "crucial".
Mr Ham said: "We think it is necessary to do because if you're not willing to do go through that process and support plans of this kind, essentially you are colluding as politicians in the continuation of unsafe services.
"So politicians need to step up to the plate and be brave.
"Not in all cases ... because there have been examples of consultations in the past which haven't been well-founded.
"But where the evidence is clear, that's where Government and local politicians need to do their job."
Mr Ham said he did not agree however, with STP proposals to slash hospital beds.
The report said any plans to reduce hospital beds should be tested "if necessary to destruction".
Mr Ham said there was "no prospect realistically" of cutting beds when this winter had shown all the beds were needed.
Last November, the King's Fund said STPs had been kept secret from the public and barely involved frontline staff.
NHS England ordered local health leaders not to reveal the plans to the public or the media until they were finalised and approved by their own officials first.
It even told local managers to refuse applications from the media or the public wanting to see the proposals under the Freedom of Information Act.
Asked if NHS England had contacted the King's Fund after that report was published, Mr Ham said: "They thought we were quite critical of the process but we stood behind the line we took because we reached the decision to highlight the weaknesses in the process based on the many interviews we'd undertaken."
He said that as an independent organisation, the King's Fund would be "falling down on the job" if it was to give in to any pressure.
A spokeswoman for NHS England said: "The King's Fund report rightly finds that STPs offer the best hope for the NHS to transform care for patients in a sustainable way. They will allow the NHS to take advantage of new technologies, adopt successful practice more widely, and make practical improvements in areas that we know matter most to patients.
"These proposals are all about putting collaboration at the heart of our care system, with health and local government working more closely together than at any time since the NHS was created. Everyone in the NHS wants help to ensure we can all get excellent care whenever we need it."
Dr Mark Holland, president of the Society for Acute Medicine, said: "In acute medicine we need to be convinced that moving resources away from hospitals will see a corresponding reduction in our volume of work. The ball is the court of STPs to convince us of ability to deliver on their promises."
Dr Mark Porter, chairman of council at the British Medical Association, said the plans were becoming "unworkable".
He said: "From the beginning, this process was rushed and carried out largely behind closed doors, by health and social care leaders trying to develop impossible plans for the future while struggling to keep the NHS from the brink of collapse."
A Department of Health spokesman said: "These NHS plans - developed by local doctors, hospitals and councils working together with the communities they serve - will help patients get better care by delivering the NHS's five year forward view, transforming mental health provision, improving cancer care, and delivering better access to GPs."