Britain’s most senior doctor has cast doubts on the sustainability of a healthcare service that is free for all. His comments come at the same time as UK Independence Party leader Nigel Farage called for a debate on the idea of replacing the NHS with an insurance based system.

Prof Sir Bruce Keogh, medical director of the NHS in England, has questioned whether the current taxpayer funded model for the NHS was “sustainable in the longer term”, adding that huge changes, including less reliance on hospitals, was needed if patients were to continue receiving free treatment.

Speaking to the Guardian, Keogh 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.”

His solution is to divert more resources and care towards GPs’ surgeries, rather than overburdening hospitals with treatments, warning that if nothing changes “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.

“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,” he added.

However, Keogh denied that the service is currently experiencing a crisis, despite figures showing that Accident and Emergency departments across Britain are struggling to meet targets. “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,” he said.

“It’s an evocative term which is also provocative and is used too freely for the wrong reasons. 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.”

Meanwhile, Ukip leader Nigel Farage has also spoken out on the NHS, warning that, although his party has “outright rejected” the idea of replacing the NHS, changing demographics make the question of how to finance the service unavoidable in the long term.

In an interview with the BBC’s Nick Robinson for the Radio 4 Series Can Democracy Work?, Farage said “There is no question that healthcare provision is going to have to be very much greater in ten years than it is today, with an ageing population, and we’re going to have to find ways to do it.”

Last November video footage emerged of Farage talking about the NHS during his 2012 Common Sense tour, in which he expressed support for an insurance-based model of healthcare to replace the NHS.

During the video he opines “I think we’re going to have to think about healthcare very, very differently. I think we’re going to have to move to an insurance-based system of healthcare.

“Frankly I would feel more comfortable that my money would return value if I was able to do that through the market place of an insurance company than just us trustingly giving £100bn a year to central government and expecting them to organise the healthcare service from cradle to grave for us.”

Speaking to Robinson, Farage said that his opinion had been over-ruled by his party, but that it was a “debate that we’re all going to have to return to.”