About 14 million UK adults, equivalent to 26% of the adult population, now regularly bank online, says analyst Gartner.
This compares with 71 million adults, or 33% of the US adult population, says the analyst.
The figures are based on a survey of over 2,000 consumers aged over 18 in the UK and the US.
Banks must now meet client service expectations on- and offline, and use these interactions to increase business opportunities, Gartner said.
"The current usage levels in both countries show that adoption is now taking place within the mainstream of consumers," said David Schehr, an analyst at Gartner.
"Banks and other financial services providers must now rethink the role of online banking within this context, and use this channel to serve their clients and strengthen relationships," he said.
Use of online banking did not result in any substantial decrease in branch use, the survey found.