There is an unspoken truth about British life: we have two classes of citizen. The first are those born or formally settled here, who have all the rights and protections of the law. Then there are perhaps a million others who may have lived here with their families for years but without the proper documents. They can be our neighbours, work in our shops, contribute to our economy — yet they do not have the same basic protections and are far more vulnerable to exploitation. These are the so-called illegal immigrants, and it is past time to offer them amnesty.

Britain has become the most successful melting pot in Europe, absorbing 2.5 million people over this decade without the far-right backlash seen in much of the continent. A recent Pew study showed that Brits are more likely than any other Europeans to say that migrants make the country -stronger. This is why the Windrush scandal was so damaging to the Tories. To deport people who have been living here peacefully for years because they did not have documentation was not just inhumane but fundamentally un-British. The same principle applies to a great many people who could be considered illegal migrants.

Consider the case of Ben James, Nigerian by birth, who was sent to school in London and then, at the age of 14, abandoned by his family. He managed to build a career and, as a successful commodities broker in his twenties, approached the Home Office in order to regularise his status. He was asked to leave Britain but fought for the right to stay, eventually winning. His case was taken up by The Spectator in 2001 when we first made the case for an amnesty for people in his situation. The editor, then, was Boris Johnson.

There is no sign that the Prime Minister has changed his mind about the need for an amnesty since he embarked on his political career. On the contrary, he made the case for this when he was Mayor of London and returned to this theme during the later stages of the Leave campaign. The dilemma he now faces is easy to understand. As this magazine argued in a cover article earlier this year, he needs to win over Brexit party voters, so he may be tempted to sound tough on migration and quietly bury his support for an amnesty. But the case is there to be made.

The main objection is that people who broke the law in coming here ought not to be rewarded. But this position overlooks the complexities of modern migration patterns and the number of people affected. Ben James, for example, was a child when he was brought here. Other ‘illegals’ have had families and found jobs; they pay taxes. David Wood, a former head of immigration enforcement at the Home Office, has estimated that there are now 1.2 million undocumented migrants in the UK — more than the population of Birmingham. For the UK government to be theoretically committed to their expulsion is an absurdity.

An amnesty would not increase the actual population of Britain (as opposed to the official population): these people are living here anyway. What it would do is bring them out of the black economy, make it more likely that they will pay tax, and give them a greater incentive to make a contribution to civic life.

There is now ample evidence to show that amnesties strengthen society. Ronald Reagan offered an amnesty to illegal immigrants in 1986, and studies show that they were then far better able to integrate into society: their language skills increased, and their wages rose by up to 25 per cent as they were able to escape the unregulated, exploitation-ridden shadow economy. A 2005 amnesty in Spain raised an extra €4,000 of tax revenue for every naturalised citizen.

An amnesty would not mean that we stop policing the borders. A commonsense line can be drawn between those who have lived here for several years, and those who have not put down roots, who can be removed in a way that deters illegal immigration. The pressure group Migration Watch UK has argued that amnesties encourage more illegal immigration — but any amnesty could require a qualification period of ten years’ residency. That is unlikely to offer much temptation. The problem is that for years UK authorities have been pursuing those who are living and working peacefully, rather than focusing on criminals and smuggling gangs. Much of this stems from a failure of politicians to talk about this rationally.

The Prime Minister is accused by his enemies of being a cynic who bends his principles to the prevailing wind. But he made the unpopular case for an amnesty as London Mayor, did not resile from it as Foreign Secretary and has made supportive noises since moving into 10 Downing Street. It would not be so radical given that, in practice, there are already routes to citizenship for many illegal migrants. The next step should be to formalise this, on generous yet practical terms, and put it in the 2019 Tory manifesto.

This would be a bold expression of the Prime Minister’s personal brand of liberal Conservatism, and of the version of Brexit he articulated in the Leave campaign. An amnesty would carry political risk, but so does any worthwhile reform. He is already persuaded of the principle. We now need the policy.