Millionaires are expected to control nearly half of the world's personal wealth by 2019, according to a new study, suggesting that the wealth gap will continue to widen. The Global Wealth report from Boston Consulting Group (BCG) said the number of millionaires in the world grew to 17 million in 2014, up from 15 million in 2013. The world's millionaires now control 41 percent of the $164 trillion in global private wealth, up from 40 percent in 2013. The report said millionaires are expected to control 46 percent of the world's wealth in 2019.

The wealthier are getting more and more wealthy Anna Zakrewski Partner and managing director, Boston Consulting Group

The growing fortunes of the wealthy are owed largely to rising stock markets and asset prices around the world. According to BCG, 73 percent of the gains in global private wealth last year came from market performance on existing assets rather than newly created wealth or businesses.

"The wealthier are getting more and more wealthy," said Anna Zakrzewski, a BCG partner and managing director. "They have a much larger share of their wealth invested in equity markets and last year was a good year for market performance." Read MoreWhy wealth inequality isn't a bad thing By far, the U.S. still has the largest number of millionaires. That segment of the population grew by 4.7 percent last year to 6.9 million. (BCG defines millionaires as households with $1 million in easily monetized wealth—cash, stock and securities, pension funds and other financial assets. Their wealth measurement doesn't include real estate, business ownership and collectible and consumer goods).

China ranked second in millionaire population but had the largest number of new millionaires in 2014. Its millionaire population grew to 3.6 million from 2.4 million in 2014, meaning the world's second largest economy added more than half of the world's 2 million new millionaires last year.

Number of millionaire households (thousands) Ranking 2014 Country 2014 2013 %-Change 1 USA 6,906 6,595 4.70% 2 China 3,613 2,425 49.00% 3 Japan 1,125 1,074 4.70% 4 UK 731 606 20.60% 5 Switzerland 461 415 11.10% 6 Canada 418 362 15.50% 7 Germany 350 338 3.60% 8 Taiwan 325 310 4.80% 9 Italy 291 271 7.40% 10 Russia 242 198 22.20% 11 France 230 225 2.20% 12 Hong Kong 229 210 9.00% 13 Australia 215 182 18.10% 14 Netherlands 187 158 18.40% 15 Belgium 164 155 5.80%

Ranking third was Japan, with 1.1 million millionaires, up 4.7 percent from 2013. Switzerland had the highest concentration of millionaires, or millionaires per capita. Fully 13.5 percent of Switzerland's population are millionaires. Bahrain ranked second, with 12.3 percent, followed by Qatar with 11.6 percent. The report also highlighted a growing divide between the rich and the super rich, as billionaires and those with hundreds of millions of dollars enjoy stronger wealth gains than mere millionaires. The number of ultra-high-net-worth households—which is defined as $100 million and up—is expected to grow by 19 percent globally and 12 percent in North America by 2019.

In contrast, the number of "lower high-net worth" households, or those with $1 million to $20 million, is expected to grow by only 6.9 percent.