Apple’s iPhone 5 launched at the end of September,  but first week sales slowed Android’s growth in Cupertino’s two biggest markets: the U.S. and U.K. during the 12 weeks ending Sept. 30, according to a new report by Kantar Worldpanel ComTech (http://www.kantarworldpanel.com), which specializes in analysis of the telecommunications field.

“While this latest data set only includes one week of iPhone 5 sales, we can see that in markets with a large number of existing Apple customers, sales have already seen a significant boost,” says Dominic Sunnebo, global consumer insight director at Kantar Worldpanel ComTech. “We expect this momentum to be fully realized in the next set of results.”

In the U.S. iOS boosted its share to 35.7%, up from 21.5% on the year-ago period , while Android’s share fell from 66.4% in the year-ago period to 57.5% this year.