The launch of Apple's iPhone 4 also marks the end of unlimited smartphone data plans from major UK mobile phone companies.
O2, which was Apple's exclusive UK partner for the iPhone, has introduced a monthly cap of up to 1GB (gigabyte) for data on new contracts.
This mirrors a similar recent move by rival Vodafone, and Three has also today announced a 1GB monthly cap.
Some observers expressed surprise at Three's decision to impose a limit.
Ben Wood, a mobile phone analyst at CCS Insight, said: "We had expected that Three would seek to differentiate its offer from rivals by offering bigger data bundles.
"It has obviously realised that this approcah would see heavy data users signing up and clogging up its network".
