Media playback is unsupported on your device Media caption Glenn Greenwald discussed the impact of the revelations on BBC Newsnight

Glenn Greenwald, a journalist who covered data leaked by US whistleblower Edward Snowden, has announced he is leaving Britain's Guardian newspaper.

Mr Greenwald said he was departing to take up a "dream" opportunity.

Documents from Mr Snowden, a former National Security Agency (NSA) contractor, revealed internet and phone surveillance by US intelligence.

Brazil, where Mr Greenwald lives, says it wants to speak to Mr Snowden over allegations the US spied on it.

According to information leaked by Mr Snowden, the NSA intercepted communications by Brazilian President Dilma Rousseff and monitored millions of emails and phone calls by the country's citizens.

As a result, Ms Rousseff cancelled a planned visit to the US.

Jose Alberto Freitas, the head of intelligence in Brazil's police force, said it was "crucial" to speak to Edward Snowden, as he could "provide technical details that will help our investigation advance".

Mr Snowden, who is wanted by the US on espionage charges, is currently living in Russia, having been granted asylum by Moscow.

'Crucial role'

In a statement posted online, Mr Greenwald described his "partnership with the Guardian" as "extremely fruitful and fulfilling" and praised the editors and journalists of the newspaper.

He said his new venture was "momentous" but he was not yet able to give any details about it.

He told online news site Buzzfeed he would be involved in a "new, large-scale... media outlet".

The Guardian's Jennifer Lindauer said: "Glenn Greenwald is a remarkable journalist and it has been fantastic working with him.

"Our work together over the last year has demonstrated the crucial role that responsible investigative journalism can play in holding those in power to account."

Mr Greenwald has been fiercely criticised for his work with Mr Snowden. But in a BBC interview earlier this month Mr Greenwald said none of the information he had published could have damaged national security.

In August, his partner David Miranda, who lives with the journalist in Rio de Janeiro, was held for nine hours at Heathrow under anti-terror laws. Police seized a number of electronic items he was carrying.

British Home Secretary Theresa May defended the move, saying the police had to act if someone had "highly sensitive, stolen information".

Mr Miranda denied wrongdoing.