Harri is joining the corporate communications and public affairs consultancy to offer senior counsel to its blue chip clients in Europe and the Middle East.
He is currently a contributing political editor of GQ magazine and a director of S4C television and The Hay Festival of Literature & Arts.
Harri left Virgin Media owner Liberty Global after serving two years as its MD of communications. Before joining in 2015, he spent three-and-a-half years at Sun and Sunday Times owner News UK, telling PRWeek on his departure that it was "job done" in terms of restoring the company’s reputation in the aftermath of the phone-hacking scandal.
Before that, he worked as then-Mayor of London Boris Johnson's spokesman and director of external affairs, and spent 18 years as a reporter and presenter at the BBC, with foreign postings to Rome and New York.
Harri said: "The challenges, energy, wisdom and experience at Hanover is not only impressive but inspiring, and I look forward to working more closely and regularly with the fabulous people I’ve recently met, and those I have known and respected for many years."
At Hanover, Harri will report to Charles Lewington, the agency’s founder and chief executive. Lewington said: "Guto is a fantastic professional who has a uniquely deep understanding of politics, media and the corporate world, backed by years of experience.
"His particular expertise in media and telecoms helps reinforce Hanover’s position as a leading consultancy for brand, employee engagement, corporate PR and regulatory support for tech businesses."
Hanover continued to climb up the PRWeek UK Top 150 consultancies table. It reached number 23 in the most recent table, as UK revenue rose 18.5 per cent to £11.7m.