Glenn Greenwald, the journalist who broke a string of stories about widespread electronic surveillance by the National Security Agency based on files leaked by whistleblower Edward Snowden, has announced that he is leaving the Guardian.

In a statement posted on his blog, Greenwald said: "My partnership with the Guardian has been extremely fruitful and fulfilling. I have high regard for the editors and journalists with whom I worked and am incredibly proud of what we achieved.

"The decision to leave was not an easy one, but I was presented with a once-in-a-career opportunity that no journalist could possibly decline.

"Because this news leaked before we were prepared to announce it, I'm not yet able to provide any details of this momentous new venture, but it will be unveiled very shortly."

A Guardian spokesperson said: "Glenn Greenwald is a remarkable journalist and it has been fantastic working with him. Our work together over the past year has demonstrated the crucial role that responsible investigative journalism can play in holding those in power to account.

"We are, of course, disappointed by Glenn's decision to move on, but can appreciate the attraction of the new role he has been offered. We wish him all the best."

Greenwald joined the Guardian in 2012 from Salon. He is a former constitutional lawyer, a best-selling author, and the recipient of the first annual IF Stone award for independent journalism.