Almost 30,000 people joined the ranks of the global super-rich last year, as booming global stock markets boosted the fortunes of the already very-rich and tipped them over into the ultra-high-net-worth bracket.

The global population of UHNW people, classed as those with more than $30m (£23m) in assets, increased by 12.9% last year to a record 255,810 people, according to a report by research firm Wealth-X.

The Wealth-X World Ultra Wealth 2018 report, published on Wednesday, said there had been a “sharp acceleration” in the number of UHNW people due to “significantly more favourable conditions for wealth creation, despite still volatile geopolitics”. The rate of growth of the UHNW population was far faster than in 2016, when it grew by 3.5%.

Collectively, the quarter-of-a-million UHNW people have a combined fortune of $31.5tn – almost double the GDP of the US.

“Buoyed by a synchronised upturn in the world economy, rising asset markets and robust corporate earnings, the combined net worth of the ultra-wealthy population increased by 16.3% to $31.5tn,” the report said. “A testament to the auspicious economic climate, the ultra wealthy population and its total net worth increased in all seven major regions.”

The fastest growing region was Asia, with China and Hong Kong becoming particularly wealthy. Hong Kong overtook New York as the city home to the most ultra-rich people, with the new total of 10,010 UHNW individuals representing a 31% increase on the previous year.

“The broad upturn in global financial markets and the reflation trade from higher interest rates were among the drivers of robust wealth gains,” the report said. “The Hong Kong dollar is pegged to the US currency so domestic rates largely followed the upward path set by the US central bank. Wealth creation was also supported by enhanced trade and investment links with mainland China, including a direct trading connection between the Hong Kong and Shenzhen bourses.”

The US is still home to the most UHNW people with 79,595, followed by Japan with 17,915 and China in third place with 16,875. The UK slipped from seventh to eighth – being leapfrogged by Hong Kong – due to “stagnant incomes and uncertainty over Brexit weighed on investor, consumer and business sentiment”.

Only 13.7% of UHNW individuals are women, but the number of women grew by 30% outstripping the 10% rate of growth for men. The study predicts that the size of the ultra-rich population will continue to increase, with the number of UHNW individuals expected to grow to 360,000 by 2022.