For many years, British voters have said that illegal immigration is among their chief concerns, and that they want the Government to be much more radical in combating it. But very few of them know either the scale of the problem – how many illegal immigrants live in Britain – or the nature of it. As a result, the idea of an amnesty creating a route to citizenship has long been widely derided. That is what makes the latest intervention of Nadhim Zahawi, a rising Tory MP, fascinating.

In a provocative essay for a think-tank called Right Revival, Mr Zahawi argues that the Tory party should embrace the idea of an amnesty as a means of reconnecting with voters from ethnic minorities. In fact, there are much better reasons for supporting such a policy.

Illegal immigrants may arrive unlawfully, may arrive lawfully but overstay, or may be the children of those who do either. Naturally, their number cannot be guessed with much precision, but estimates are generally about 600,000 at least. There are three policy approaches to this problem.

First, mass deportation; second, the status quo; and third, amnesty. The first won’t happen; the second is ineffective; and the third has costs and benefits. One cost is that it might be seen to reward illegal behaviour; another is that it would encourage further migration. The evidence for both is limited. The benefits, however, are clear. The London School of Economics has estimated that taking illegal immigrants – many of whom have been here for more than a decade – out of the shadows could reduce criminality and generate £3bn in taxation.

It was Citizens UK, the community organising group, that put an amnesty on the political agenda. Since it began campaigning in the last parliament, some surprising advocates have emerged, including Boris Johnson. Smart Tories are realising that while the word amnesty is thought to imply a soft touch, in fact the moral, social and economic case is hard-headed. As Mr Zahawi notes, it may be good politics, too.