Almost half of UK internet users are going online via mobile phone data connections, according to the Office for National Statistics.
Some 45% of people surveyed said they made use of the net while out and about, compared with 31% in 2010.
The most rapid growth was among younger people, where 71% of internet-connected 16 to 24-year-olds used mobiles.
Domestic internet use also rose. According to the ONS, 77% of households now have access to a net connection.
That figure was up 4% from the previous year, representing the slowest rate of growth since the ONS survey began in 2006.
Among the 23% of the population who remain offline, half said they "didn't need the internet".
The ONS report is the first since dot-com entrepreneur Martha Lane-Fox was appointed as the government's UK Digital Champion, with a brief to increase internet uptake.
In a statement, Ms Lane-Fox said: "That so many offline households don't see any reason to get online reinforces the importance of the digital champions network that the Raceonline2012 partners are creating."
The figure for domestic connections contrasted sharply with the rapid growth in uptake of mobile services.
However, the popularity of 3G broadband did not necessarily mean that more people were going online overall.
Many of those using mobile phones are likely to already have home broadband connections.
Older users, who the government is particularly keen to get connected, appeared to have been relatively untouched by the phenomenon.
While 71% of 16 to 24-year-old who went online said they used mobile broadband, just 8% of internet users aged over 65 made use of the newer technology.
The ONS survey also found a dramatic rise in the use of wifi hotspots - a seven-fold increase since 2011 - suggesting that the rise of 3G has done little to slow demand for free and paid-for wireless access.
All findings were based on a monthly survey of 1,800 randomly selected adults from across the UK.