“A no-deal Brexit … is expected to have an immediate and drastic effect on supply chains” for medicines. From the pen of an op-ed writer, such language might seem alarmist. But this comes from a report published this week in the Lancet, a world-class scientific journal – and it should terrify you.

The scale of the problem is huge. You will know someone affected, even if you are not. NHS figures show that almost half the population regularly takes a prescribed medicine. And around 75% of the medicines the NHS uses come into the UK from the EU.

But what we don’t know is which medicines will be affected. The secretary of state for health, Matt Hancock, knows. The Commons health and social care select committee asked him to publish a list of the medicines and medical products for which his department had supply concerns. But despite the public interest, he refused, citing “commercial confidentiality”. And he’s muzzled suppliers with non-disclosure agreements so they can’t tell us either.

It’s not just drugs. The Medical Schools Council – the representative body for UK medical schools – gave evidence that 3.7 million people in the UK have been diagnosed with diabetes, yet less than 3% of insulin is manufactured in the UK. “Any disruption to the supply chain could have catastrophic consequences,” they added, before comparing a no-deal Brexit to the break-up of the Soviet Union in the 1990s, when deaths from diabetes rose ten or twenty fold.

The government has sought to reassure patients that its contingency plans are robust, but the Lancet report says “stockpiling arrangements cannot cope for more than a few weeks”. The Lancet also notes that some affected products, such as radioisotopes needed for treating some types of cancer, simply cannot be stockpiled.

How the government plans to deal with shortages was the subject of a brief, private consultation last year. What it suggested was a “serious shortage protocol” dealing with “in particular … a scenario with multiple large shortages”. Those are words that should alarm you – which may well be why the government has not used them in any of its public documents.

‘Matt Hancock has muzzled medical suppliers with non-disclosure agreements.’ Photograph: Jonathan Brady/PA

Subsequently, it has legislated for serious shortage protocols. The government, if or when shortages arise, will now permit pharmacists to substitute a different “strength, quantity or pharmaceutical form of a prescription-only medicine, or a different prescription-only medicine” to that prescribed by your doctor. No longer would your medicines be prescribed by the doctor who knows your medical history – instead, it would be a pharmacist acting in accordance with a protocol drawn up by the government.

These measures break public promise the government has made: “If there are any shortages of particular medicines after EU exit, the same system will be in place – your doctor will advise you of the best alternative to treat your condition.”

This latest legislation has alarmed medical organisations, some of whom did not even know of the proposals. The National AIDS Trust, for example, has said: “We are deeply concerned that these changes were made without proper consultation. Prescribing HIV medication is a complex process which must take account of a multitude of factors. The only person qualified to safely alter the medication prescribed to a person living with HIV is that person’s HIV consultant.”

Staggeringly, the government seem to be driven by a desire to save costs on a no-deal Brexit. Its own explanatory notes state that: “The main benefits of the protocol would be the NHS cost savings associated with GP time” – even though, as if it were mere afterthought, “there may be some risks to patients.”

We at the Good Law Project believe that there is another big problem with serious shortage protocols: they are unlawful. Last week we wrote to the government threatening urgent judicial review proceedings if they refused to withdraw the power to make them.

How should the government respond to such a threat? It seems unthinkable that it might take the country over the no-deal cliff without knowing whether it can use a key measure to try and alleviate harm to the health of millions of people. It has even recognised in correspondence that there are “potentially serious public health implications” if SSPs cannot be used. So how, in such circumstances, can it contemplate no deal?

We think it’s time the government came clean. Serious shortage protocols could be dangerous and may well be unlawful. No responsible government should expose its own people to such risks. We need to delay Brexit – and plan properly.

If you wish to support Good Law Project’s judicial review of serious shortage protocols, you can do so here.

• Jolyon Maugham QC is a barrister and director of the Good Law Project