For a city with an ageing population, a baby boom should be good news. But the one under way in Hong Kong is the result of women travelling across the border from mainland China to give birth in the city, and it is provoking an angry backlash.

The number of babies born in the city to mainland Chinese women who were not married to Hong Kong husbands rose to 32,653 last year – more than a third of all the city’s births – and up from 620 a decade ago.

Most mainland mothers-to-be are drawn by the prospect of permanent residency for their children, an automatic right for those born in Hong Kong, which retains a much more liberal political, legal and economic system than the mainland. There is also the added benefit of avoiding a financial penalty for breaking Beijing’s one-child policy.