The UK’s ranks of the ultra-rich have swelled by 400 over the last year, taking the number of people with fortunes of more than $50m (£38m) to nearly 5,000.

The fortunes of the already very wealthy are growing at a far faster rate than the general population, according to a report by the Swiss bank Credit Suisse published on Thursday.

The number of ultra-high net worth individuals (UHNWIs) in Britain over the 12 months to summer 2018 increased by 8.5% to 4,670, while the average Briton saw their wealth, including property, increase by just 1% to £213,000.

Anthony Shorrocks, an economist and author of the report, said the fortunes of the rich were growing much faster than the general population because those with investments had benefited from booming stock markets. “There are a lot of people with wealth just below $50m, and they are tipping over into the UHNWI bracket,” he added.

The richest person in the UK is Sir Jim Ratcliffe, the founder and chief executive of the petrochemicals company Ineos and a high-profile Brexiter, who has an estimated fortune of £21bn. Ratcliffe is preparing to leave Britain for tax-free Monaco – just months after he was knighted by the Queen for services to business and investment.

While the number of the UK’s ultra-rich is growing quickly, the pace in the US is even faster. More than 6,000 in the US joined the UHNWI bracket, taking the total to 70,540, which means the country accounts for 47% of the global UHNWI population. The report said there was a “seemingly relentless rise in household wealth in the United States”.

China is in second place with 16,510, followed by Germany (which an 11% decline) with 6,320. The UK has the fourth-largest population of UHNWIs, ahead of Japan, India and Italy.

Globally, there are just under 150,000 people with assets of more than $50m. Of these, 50,000 have personal wealth of more than $100m, and 4,390 have assets of more than $500m. The total wealth in the world increased by $14tn to $317tn.

The number of UHNWIs has increased fourfold since the turn of the millennium, according to the report. “This is due in part to the fact that the millionaire and UHNW boundaries are static and absolute, while the whole distribution of wealth is shifting as the world becomes a wealthier place, progressively lowering the bar for membership over time,” the report said. “Rising inequality can also boost the speed at which new millionaires are created.”

The richest 1% own just under half of the world’s global assets, which is roughly the same proportion as last year. Credit Suisse said the dominance of the super-rich had been driven by rising inequality, but there are signs the situation may be improving.

“Since the global financial crisis, wealth inequality has trended upward, propelled in part by the rising share of financial assets, and a strengthening US dollar,” the report said. “These underlying factors appear to be waning, so that it seems more likely that wealth inequality will fall in the future rather than rise.”

The report found women are beginning to catch up with men in terms of assets, with females holding 40% of global wealth.

However, there are still very few female billionaires. Just 244 out of the 2,208 billionaires listed on Forbes magazine’s rich list are female. Germany has the highest proportion of female billionaires, with one in four of people with nine-zero fortunes being women.

Credit Suisse highlighted Indonesia, Singapore and Taiwan as countries with more than 20 male billionaires and no female equivalents.

Nannette Hechler-Fayd’herbe, the bank’s head of investment strategy and research, said: “Although there remains a wealth gap between men and women globally, in some places much more marked than others, that gap has narrowed significantly over the years and is expected to continue doing so as more women access education and participate in the labour market.

“There are also signs that more self-made women are succeeding in business and entering the highest wealth ranks. However, even in the countries where progress is the strongest, challenges remain. More needs to be done to ensure that women have an equal opportunity to build up, inherit and share in wealth.”