The United Kingdom's National Health Service might be forced to abandon the concept of free healthcare for all if it doesn't undergo radical change, according to Britain's most senior doctor.

The publicly funded healthcare system will "not be fit for the future" unless it becomes far less reliant on hospitals and undergoes a "complete transformation" of the way it currently operates, said Professor Sir Bruce Keogh, top medical director of NHS England, said.

"If the NHS continues to function as it does now, it's going to really struggle to cope because the model of delivery and service that we have at the moment is not fit for the future," he told the Guardian, adding that general practitioner surgeries need to be given more resources to cope with the pressures.

If the NHS is to withstand increased demand and tightening budgets, more services need to happen under the same roof, such as diagnostics tests and an expanded range of treatments at GP surgery, The Telegraph reported.

"If not, we will get to a place where the NHS becomes unaffordable and we will have to make some very difficult decisions which will get to the very heart of the principle of the NHS and its values," Keogh said.

"This will open up a whole series of discussions about whether the NHS is fit for purpose, whether it's affordable, and whether the compact with the citizen of free healthcare for all is sustainable in the longer term."

"Too many patients find the NHS fragmented (and) confusing. They find that they get pushed from pillar to post; they feel like a ball in a pinball machine at times," he added.

Over the winter, three of the 10 ambulance trusts in England declared a critical alert, with accident and emergency waiting times deemed to be at their worse levels in a decade.

But Bruce denied claims that ambulance services, A&E and GP surgeries are struggling to cope and are going through a crisis, UK MailOnline reported. "Everybody that's working out there in the NHS knows that they're under a lot of pressure at the moment. They don't like the term 'crisis' being applied willy-nilly."

"It's an evocative term which is also provocative and is used too freely for the wrong reasons," he continued.  "It's a period of unprecedented pressure, of undue pressure. But the NHS is facing very difficult times, yes. The word 'crisis' implies that you can't deal with it."

Additionally, even though the frontline services "are going through a critical phase," they will recover.

Currently, it remains doubtful whether the taxpayer-funded model will be "sustainable in the longer term," he said.

"In common with healthcare systems around the world, the NHS is facing unprecedented demand, but undermining the principle of services being free at the point of use is not the answer," a Department of Health spokesman said.

"Instead, we are backing the NHS's plan for the future and have provided an extra [2 billion pounds] in funding next year to transform out-of-hospital care and meet the needs of an ageing population."