LONDON (REUTERS) - Alan Rusbridger, editor of Britain's Guardian newspaper, who helped break news of widespread surveillance by the US National Security Agency (NSA) based on the leaks of analyst Edward Snowden, will step down next summer, its parent company said on Wednesday.

Rusbridger, 60, who has been editing the left-leaning newspaper for nearly 20 years, will become chairman of the company's Scott Trust, which safeguards the paper's editorial independence, in 2016.

The Guardian and the Washington Post won the prestigious Pulitzer Prize this year for their reporting on the NSA's surveillance activities.

Last year, British Members of Parliament accused Rusbridger of breaking the law by publishing details about intelligence services that they said could play into the hands of potential attackers.

Under Rusbridger's editorship, the Guardian also played a key role in uncovering Britain's phone-hacking scandal which eventually led to the closure of Rupert Murdoch's best-selling News of the World newspaper and the jailing of one of its former editors Andy Coulson.

Rusbridger said in a statement on Wednesday: "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."

Parent company the Guardian Media Group said the Scott Trust will soon announce how it will appoint Rusbridger's replacement.