The drive behind the growing power of the state is not power-grabbing politicians, but popular demand. As the technical and administrative capacities of the state expand, so people demand more of it in their constant pursuit of security. The state must, if it can. Sometimes the state must, even if it cannot.

The coronavirus pandemic is a perfect illustration. As government experts acknowledge, whatever social distancing is imposed, there is serious doubt about whether it is possible in the long term to suppress this virus without a high degree of herd immunity.

In the absence of an effective vaccine, that would require large-scale exposure to the disease of a kind that killed many people in the Spanish flu pandemic of 1918-20, but ultimately put an end to it. On that occasion limited social distancing measures were adopted in parts of the US, but not in most countries, including Britain.

Things are different today, but the difference is not wholly benign.

Public pressure for action at whatever cost pushes the measures beyond what they can realistically expect to achieve. It may well push them beyond what is worth achieving if the price is the destruction of our personal liberty, livelihoods and sociability. There are dissenting voices, but not many and they are drowned out in a torrent of collective emotion and abuse.

Coronavirus is not unique in this respect. It is one of several external threats that has provoked a big change in the relations between citizens and the state. Others include terrorism and climate change. Their effect has been to enhance our dependence on the state and to devalue effective parliamentary scrutiny. Parliament tends to be regarded as a mere impediment to effective action. This is a profound change in our political culture.

The prime minister’s orders on Monday night are a remarkable example. The Coronavirus Bill had only just been introduced into parliament. It would, when passed, confer draconian powers on ministers to control the “gathering” of any two or more people anywhere and to restrict a person’s right to enter of leave any “premises”, including that person’s home or car . . . or tent. However, in his press conference Boris Johnson purported to place most citizens under virtual house arrest through the terms of a press conference and a statement on the government website said to have “immediate effect”. These pronouncements are no doubt valuable as “advice”, even “strong advice”. But under our constitution neither has the slightest legal effect without statutory authority.

At the time of writing (Wednesday morning), it is unclear what power the prime minister thought that he was exercising. The relevant powers of the government are contained in the Public Health (Control of Disease) Act 1984 and the Civil Contingencies Act 2004. But it is doubtful whether either authorise the prime minister’s orders, which is presumably why the Coronavirus Bill has been introduced.

The ordinary rule is that a person may not be detained or deprived of his liberty without specific statutory authority. The 1984 act contains powers to restrict movement, but they are exercised by magistrates and apply only to particular people or groups who have been infected or whom they may have infected. The Civil Contingencies Act confers a temporary power of legislation on ministers that is exercised in a national emergency, but no specific power to detain people at home.

In the present national mood the prime minister’s orders will probably have strong public support and people will be inclined to comply whether they are binding or not. Yet we are entitled to wonder what kind of society we have become when an official can give orders and expect to be obeyed without any apparent legal basis, simply because it is necessary.

There are wider problems about this. Legislation couched in general language can be used for purposes far removed from the original intention. The terrorism legislation, for example, has been used for a variety of other more questionable purposes, ranging from blocking the deposits of insolvent banks in the interests of creditors to manhandling peaceful demonstrators on the streets of London.

Governments armed with vast powers are usually reluctant to part with them. The wartime defence regulations, which required the population to “place themselves, their services and their property at the disposal of His Majesty”, had to be renewed annually, but were not allowed to expire until 1964.

Other wartime powers were expressed to continue until the government declared the war to be over, which it never did. They continued to be used until the 1980s, when the Scott report exposed this unsatisfactory corner of governmental practice.

These are not just technicalities. There is a difference between law and official instructions. It is the difference between a democracy and a police state. Liberty and the rule of law are surely worth something even in the face of a pandemic.