Britain’s NHS faces a funding gap of £20-30billion over the next three years.

Rising demand and rising costs dictate inevitable questions about how much more money we need to spend on health services and how it is raised.

Governments tend to boast that they are spending more on the NHS than ever before. As costs rise, increased resources are necessary simply to stand still, so such boasts are true in absolute terms. But in reality, two Conservative governments have presided over eight years of historically low increases in NHS funding,

At present, the UK spends a similar proportion of our GDP on health as many comparable European countries, but achieves less. We have fewer doctors, nurses and beds than many of our neighbours. We have fewer medical imaging scanners and patients face longer waiting times. It is true that the NHS in Scotland has not suffered as badly as it has in England, but it faces the same pressures.

Medical advances, ironically, contribute to the problem. We are all living longer, but often with costly chronic complaints. Sophisticated drugs and treatments are able to be better adapted to the needs of individual patients. Success is expensive.

Increased taxation in some form is the answer proposed by experts, including Dr Mark Hellowell of Edinburgh University, writing in the BMJ today. They suggest this could be some form of supplementary income tax, or by earmarking taxes on products such as cigarettes, junk food or sugary drinks.

Such suggestions are beguiling. Nobody likes tax increases, and polls have regularly shown a willingness among the public to pay more if it were guaranteed to go to the health service. But such ring-fencing of tax, known as hypothecation, is a false and risky proposition.

A risky proposition, because this is a slippery slope. If other areas of public spending need extra funds, will there be more such taxes? What happens to less popular areas of spending – from overseas aid, to support for the drug-dependent? They will inevitably be squeezed.

Some doubt the value of politicians, but one of their key roles is to take informed decisions balancing different pressures on the public finances based on the best available evidence. This worth is lost, if their room for manoeuvre is increasingly restricted by ring-fencing.

And it is a false proposition – because such pledges are inevitably broken. Promising the tax on sugary drinks will be spent on work in schools is the latest, and in time it will inevitably go the way of the “road tax” which is in no way linked to spending on the roads.

Putting more money into the NHS should, of course, be a priority. But ring-fencing is neither the most efficient nor the fairest way of funding the NHS. It should come from general taxation.