Those worth less than $1 million will see their share of wealth shrink to 29 percent of the total, from 34 percent in 2015 and 40 percent in 2010.

Still, millionaires currently control 47 percent of the world's wealth, and will control 52 percent by 2020, according to the report. And the richest of the rich will gain the most, especially in the U.S. Those worth more than $20 million in the U.S. will control 29 percent of the country's wealth by 2020, up from 24 percent in 2015 and 20 percent in 2010, according to the report.

The number of millionaire households in the world grew 6 percent in 2015, to 18.5 million, according to the Boston Consulting Group Global Wealth report. The increase marked a slowdown from the 11 percent millionaire growth in 2014.

Millionaires will control more than half of the world's wealth by 2020, according to a new report, with the richest millionaires gaining the most.

The trend will likely accelerate in the next five years, as the largest wealth gains are expected to go to those at the very top of the top. U.S. households worth $20 million or more will see their wealth growth by more than 7.6 percent over the next five years, according to the report. Those in the U.S. worth $1 million to $20 million will see there wealth grow 5.7 percent, while those worth less than $1 million will see their wealth grow 1.2 percent.

The strongest growth in global millionaires in 2015 was in Asia, excluding Japan, which saw its millionaire household population grow 17 percent to 3.6 million, followed by Western Europe with 11 percent growth to 3.9 million. Latin America's millionaire population grew by 7 percent to 400,000.

North America saw the slowest growth of any region, with its millionaire household population growing less than 1 percent, to 8.4 million. That marked a slowdown from the 9 percent growth in 2014.

Yet the U.S. still has far and away the largest number of millionaires, with 8 million. China ranks second with 2.1 million, followed by Japan with 1.1 million and the United Kingdom with 961,000.

The country with the highest millionaire households per capita — known as millionaire density — is Liechtenstein, where 19 percent of households are millionaires. Switzerland ranked second with 15 percent, followed by Bahrain with 13 percent and Qatar with 12.7 percent. The U.S. ranked ninth, with 6.5 percent of its households being millionaires.

The total private wealth in the world grew by 5.2 percent in 2015, to $168 trillion. That marked a slowdown from the 7 percent growth in 2014.

In the next five years, wealth is projected to become even more highly concentrated at the very top — and increasingly only at the very top of the top.

While wealth growth in 2015 was driven mainly by cash and deposits, given the weak stock market, wealth in the next five years will be driven by equities, the report said. Of the nearly $15 trillion that's expected to be created by 2020, nearly $14 trillion will come from equities, which will account for 69 percent of global wealth in 2020, up from 62 percent in 2015 and 54 percent in 2010.