iPhone users are downloading the new iOS 7 in large numbers, ramping up the pressure on developers whose apps are struggling to cope with the upgrade.

After 24 hours of availability, online advertising network Chitika found that 18 per cent of traffic on its network in North America was generated by iOS 7, three percentage points higher than iOS 6 on its first day.

Mobile web and app analytics provider Mixpanel found that 35 per cent of iOS-powered devices reaching its network were running the new version. The company estimates that iOS 7 activity could exceed iOS 6 today (20 September).

But app testing company uTest has warned that many app developers aren’t up to speed with optimising their programs to work with the new version of the OS.

Speaking to Business Insider, the company’s chief marketing officer Matt Johnston said 90 per cent of iOS apps tested for the first time on iOS 7 by uTest are “having trouble”, with problems including crashes and incorrect font sizes that cut off text.

In addition, iOS 6 apps that use the swipe-up function will have problems as the gesture isn’t available in iOS 7, because this now provides access to the Command Center for device settings.

Apps are apparently having performance issues in twice as many areas as they typically do with an iOS upgrade, while developers are taking three or four attempts to fix issues created by iOS 7, it was said.

Tim Cook, Apple’s CEO, described iOS 7 as “the biggest change to iOS since the iPhone”, suggesting compatibility issues were likely. The scale of the change has clearly caught some developers out meaning that the number of problems is markedly higher than for previous updates.

Wireless speaker manufacturer Sonos warned users earlier this week that it has “encountered some issues” with its Sonos Controller for iPhone app when running on the newly released version of iOS7.

The company said users who have updated to iOS 7 may find that the app closes unexpectedly when they add a new component or discard an alarm. In addition text may not be fully displayed on iPad screens and there could be “a minor user interface issue” with the play/pause button.

The company plans to release an update to fix the problems towards the end of the month.

Some of the biggest app names have updated their iPhone apps to coincide with the release of iOS 7. Facebook, Twitter, Yahoo, Evernote and Pandora are among those to have revamped their apps.

Along with the introduction of iOS 7, Apple has doubled the maximum size of app that iPhone users can download when not connected to a Wi-Fi network, from 50MB to 100MB.