Ofcom chairman David Currie will act as executive chairman and lead the board and the agency that looks after Ofcom policy until a successor is found.
"There is never a good time to leave a great job," said Carter. "However, Ofcom is now firmly established, broadband and digital competition are delivering real results, and the recent extension of David's term makes for an orderly transition."
Carter will continue to work on all operational and financial matters until 1 August, but will not be party to Ofcom's economic, competition and policy decisions after that time.
"Stephen took on an immensely challenging task and has performed outstandingly," said Currie. "His legacy is an effective and credible organisation which plays an important role in delivering greater choice, lower prices and greater innovation."
The post will be advertised on 18 June and Ofcom expects to name Carter's successor by early autumn.
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