The agency said he "will be taking a short period away from the firm to provide support to Boris Johnson", returning "later this year" as senior counsel, strategic communications.
Morris joined Edelman in 2017 from Washington DC-based political consultancy GDR. He is a former pollster, policy advisor at the Prime Minister’s Strategy Unit and was speechwriter to Ed Miliband when he was Secretary of State for Energy and Climate Change.
Hugh Taggart, Edelman general manager, said: "We’re delighted that James has taken on the role of managing director of the public affairs team. He is a brilliant strategist with a powerful vision for the team that will take our work to the next level. His leadership will see an evolution of our approach to drive even more integrated campaigns that deliver even greater impact.
"I want to thank Will for his commitment to the public affairs team; we’re looking forward to his return later this year."
Morris said: "Edelman’s deep understanding of the drivers of trust is increasingly important as we help clients tackle big societal and regulatory issues that affect their business. I’m looking forward leading the team and drawing on the breadth of insight and campaign capabilities here, alongside traditional public affairs expertise, to help our clients succeed."
Yesterday PRWeek revealed that Edelman, the UK's biggest PR agency, has named a new leadership team for its healthcare practice, comprising three women.
Other members of Johnson's campaign team include Sir Lynton Crosby, the high-profile strategist who worked with the leadership frontrunner in his bid to be London mayor in 2008 and 2012; former Conservative MP James Wharton, his campaign manager; Oliver Lewis, a director at Hanbury Strategy, who is Johnson's policy chief; and press officer Lee Cain, formerly of Vote Leave.