American intelligence agencies have been accused of spying on the emails of millions of Americans, including those of former president Bill Clinton.

In the latest in a series of intelligence scandals to hit Washington, details of a secretive email surveillance scheme are beginning to come to light - with fresh allegations reported in the New York Times.

The Times quotes one anonymous NSA analyst who claims that electronic messages sent to and from American citizens, and says that the former president - whose wife is now the country's secretary of state - was among those targeted by the sweep.

The database system, called Pinwale, is used by America's National Security Agency to intercept and examine huge volumes of email passing through American telecommunications networks.

The NSA has confirmed that Pinwale exists, although it will not comment on the latest allegations or give further details on how the system operates.

The head of the Senate Intelligence Committee, which has been investigating the unauthorised surveillance claims for several years, reacted to the news of Pinwale system by suggesting that nothing illegal had taken place.

The news is just the latest in a long series of revelations about the extent to which America's security agencies are keeping track of ordinary people, including the controversy over warrantless wiretaps.

However Californian Senator Dianne Feinstein, a Democrat, said she had previously investigated Pinwale and believed it did not violate the law.

"We asked the questions. We were assured it was not correct," Feinstein told a Judiciary Committee hearing. "I've gone over this chapter and verse. I do not believe that any content is reviewed in this program."

That stance is a sharp contrast with four years ago, when Feinstein told the Senate said that she had a "very heavy heart" after learning that intelligence services had acted in contravention of laws that she had helped pass.

In 2005 it emerged that President Bush had bypassed the usual process of court approval for wiretaps, encouraging NSA officials to conduct wiretaps at his command.

Accused of abusing his powers, Bush later claimed it was his "constitutional responsibility" - but while Congress strongly objected, the controversy ended last year with a compromise that effectively approved his actions and gave immunity to American telecoms companies for their role in aiding the NSA.

"Ordinary Americans' most private emails have been and still are being intercepted in bulk and then stored in secret NSA databases, without probable cause," said Kevin Bankston, a lawyer with the campaign group Electronic Frontier Foundation.

The organisation, which is suing the government over the illegal interception of communications, said systems like Pinwale were exactly what it intended to stop.

"One of the remedies we're asking for in that case is the destruction of the domestic communications and records that the NSA has been illegally hoarding in databases like Pinwale."

While some of the most high-profile episodes of covert government surveillance have taken place in America, it is far from alone in monitoring the activities of its citizens.

Indeed, the prevalence of internet communication has encouraged governments and intelligence agencies around the world to focus. In the last week China has been forced to drop plans to make it mandatory to install surveillance software on every PC in the country, while the Iranian authorities have clamped down on internet communications in the wake of its disputed elections.

The UK government, meanwhile, intends to create a series of databases keeping track of every phone call, email and text message in Britain.

Earlier this year eavesdropping agency GCHQ denied that it is building its own equivalent to Pinwale, after reports that the agency had already been allocated £1bn to build a system to monitor all internet use in the UK.

The news in America, however, comes just weeks after President Obama said he would create a new office for cybersecurity - closely linked to the NSA - while vowing not to endanger people's privacy.

"Our pursuit of cybersecurity will not - I repeat, will not include- monitoring private sector networks or Internet traffic," he said. "We will preserve and protect the personal privacy and civil liberties that we cherish as Americans."