After 20 years at the paper Rusbridger will leave next summer and replaces Liz Forgan as chair of the Scott Trust in 2016

Alan Rusbridger is to stand down as editor-in-chief of the Guardian after 20 years, leaving the news organisation to become the chair of its ultimate owner, the Scott Trust.

He will stand down next summer after his successor is appointed and will replace Liz Forgan in 2016 at the end of her term as the chair of the trust, the body that safeguards the title’s editorial future and independence.

Earlier this year, the Guardian was awarded the highest accolade in US journalism, the Pulitzer prize for public service, for its exposure of the surveillance activities of the US National Security Agency, following the leaks of Edward Snowden. It was also named newspaper of the year and website of the year at the UK Press Awards.

Under Rusbridger’s editorship, the Guardian has been transformed from a British newspaper into a global voice, with one of the most-read English language news websites in the world. In October theguardian.com had 111.5 million unique browsers.

Early in Rusbridger’s tenure, the Guardian fought a successful libel battle against the former Conservative minister Jonathan Aitken, who later went to jail for perjury; and saw off legal action from another Tory MP, Neil Hamilton, in the “cash for questions” affair. More recent high-profile stories also include a long-running series of investigations revealing that phone hacking was widespread at the News of the World, reports that ultimately led to the Leveson inquiry into press standards.

The Guardian also published the contents of secret US diplomatic cables in partnership with WikiLeaks, the contents of which embarrassed governments around the world; the story was later turned into a film, The Fifth Estate, with Peter Capaldi playing the Guardian editor.

In making the announcement, Rusbridger said: “In global journalism, there are a handful of roles that have the capability to redefine our industry. I am privileged to have held one of those roles for 20 years, a period in which successful newspapers have become global content providers, reaching audiences in dramatically new and valuable ways.

“I am honoured to succeed the quite brilliant Liz Forgan as chair of the Scott Trust, preserving the independent editorial values and the long-term financial stability upon which our future depends. We have strong future leaders in place with unparalleled news and digital experience, and I know that our journalism will be in the best possible hands.”

Summing up the industry changes during his tenure,Rusbridger said: “It’s been quite an extraordinary period in the life of the Guardian. In February 1995 newspaper websites were, if they existed at all, exotic things: we were still four years off launching Guardian Unlimited. Since 1999 we’ve grown to overtake all others to become the most-read serious English language digital newspaper in the world.

“Each editor is told – this is literally the only instruction – to carry the Guardian on ‘as heretofore’. That means understanding the spirit, culture and purpose of the paper and interpreting it for the present. All that is only possible because of the unique Scott Trust, set up in 1936 to ensure the Guardian survives in perpetuity.”

In the past 12 months, the Guardian’s journalism following the Snowden reporting has also been recognised with an Emmy, the Special Award at the European Press Prize, a Polk award, the Right Livelihood Award in Sweden, and scoop of the year at the 59th Walkley awards for excellence in journalism.

As chair of the Scott Trust Rusbridger will continue to play a pivotal role in ensuring the perpetuity of Guardian journalism. Forgan said: “Alan has been the outstanding editor of his generation. Fully embracing the opportunities of the digital age, he has built on the best traditions of his distinguished predecessors, transforming the Guardian from a print-only national newspaper into the world’s leading quality newspaper website.

“We are delighted that the Scott Trust and the wider group will continue to benefit from his experience, overseeing the independent body that guarantees the editorial integrity and commercial future of the Guardian.”

Tom Watson, the Labour MP who campaigned against phone hacking, tweeted: “Very sad to see @arusbridger is standing down. It’s hard to think of the Guardian without his curious mind and softly spoken tenacity.”

Rusbridger, who will be 61 at the end of this month, joined the paper in 1979 from the Cambridge Evening News. He left to be television critic of the Observer and Washington editor of the shortlived London Daily News, before returning to the Guardian. He launched Guardian Weekend and the G2 features section before becoming editor. As editor, he changed the newspaper from a broadsheet to its Berliner format.

He is also the author of three children’s books and wrote Play It Again about playing the piano.

Succeeding Peter Preston in 1995, he became only the 10th editor of the title since its foundation as the Manchester Guardian in 1821. The process by which the Scott Trust will appoint the new editor-in-chief will be announced in due course. Before Forgan, the late Hugo Young, the distinguished columnist, had been chair of the trust.