A majority of Americans use their mobile phones to access the Internet, and a growing number use their handsets as their primary online device, a survey showed Monday.

The Pew Research Center survey found 63 percent of mobile phone owners now use their phone to go online. And because 91 percent of Americans now own a cell phone, this means that 57 percent are "cell Internet users."

The proportion of cell owners who use their phone to go online has doubled since 2009, Pew found.

The survey also found 34 percent of these mobile Internet users use their phones as their primary Internet device, instead of a desktop, laptop or tablet computer.

These "cell-mostly Internet users," account for 21 percent of the total mobile phone owner population, and are most likely to be young adults, non-whites, and those with relatively low income and education levels, the survey found.

Prior Pew studies have found some 56 percent of US adults own a smartphone, which offers easy Internet access.

The latest report found 85 percent of mobile phone owners in the 18-29 age ground use their handset to go online. That compared with 73 percent in the 30-49 age group and 51 percent in the 50-64 age category.

And in the 18-29 age group, nearly half of those with mobile phones say that is their primary device for the Internet.

Some 74 percent of African-American mobile phone owners are cell Internet users, as are 68 percent of Hispanics with mobile phones, the survey found.

Those with lower incomes were far more likely to use their phone as their primary Internet device: this was the case for 45 percent of those with an annual income of less than $30,000, compared with 27 percent of those with $75,000 or more.

The report was based on a survey of 2,252 adults from April 17 to May 19, with a margin of error estimated at 2.3 percentage points for the full group and 2.5 percent for the group using their phones to go online.