Legislation governing the collection of communication data by Britain's eavesdropping agency, GCHQ, is complex, the senior official responsible for its oversight has said .

"It is an extremely difficult act of parliament to get your mind round," said Sir Anthony May, the interception of communications commissioner.

He was giving evidence to a Commons home affairs committee that is nominally investigating counter-terrorism but has broadened its inquiry into the intelligence services, in particular the impact of the Edward Snowden leaks.

Snowden revealed the scale of GCHQ mass data collection, working in partnership with America's NSA to hoover up phone calls, emails and social networking activities. GCHQ and the NSA have repeatedly defended themselves by saying they are operating within the law, in the case of the British agency the Regulation of Investigatory Powers Act 2000 (Ripa).

Asked by the home affairs committee if Ripa needed to be changed, May replied: "That is a very big question." He is preparing a report and hinted that he would propose greater openness.

The number of requests from all public authorities, including the intelligence services and law enforcement organisations, to acquire communications data amounted to 570,000 a year. He described this as "a very large number, possibly too large".

He said the language in parts of Ripa could do with clarification but did not think parliament should waste its time doing that. If parliament were to embark on new legislation, the language could be clarified at that point. Giving evidence to the committee earlier, the former GCHQ head Sir David Omand insisted that Snowden was not a whistleblower and that there had been alternate avenues available other than leaking the documents to journalists. A true whistleblower would have exhausted the alternative avenues, Omand said.

He suggested that Snowden could have gone to Congress, flanked by the Guardian editor Alan Rusbridger and the New York Times editor Jill Abramson and created "a political stink". He added: "In my view, he is not a whistleblower, Rusbridger gave evidence to the committee before Christmas."

Omand dismissed as "innuendo" a recent report by the US network NBC suggesting that GCHQ had developed "dirty tricks".

• This article was amended on 12 February 2014. A headline, subheading and text was changed to clarify that there were 570,000 requests to acquire data and these relate to all public authorities, including the intelligence services and law enforcement agencies. It was further amended on 13 February to delete the MoD from the list of public authorities who have authority to acquire communications data under Part 1 Chapter 2 of RIPA.

Privacy poll

A large majority of British citizens have concerns about the way in which governments collect information about them, according to an international survey which shows strong support for online privacy.

The Ipsos Mori poll, published in conjunction with a debate on privacy at King's College London, found that 68% of those polled in Great Britain have concerns about the way in which governments collect information about them when they go online. An even higher number (76%) are concerned about the way in which companies collect information about them.

The concerns about the use of data are shared across the world according to the poll which surveyed 16,167 adults last October in twenty countries. A large majority of US citizens (71%) have concerns about the way in which the government collects data. This compares with 72% in France and 60% in Germany. Sweden is the only country in the survey where under half of those polled (43%) have concerns about the government's use of data.

The poll, which surveyed around 1,000 people in each country, found that 62% would rather keep their online activities private even if that would mean losing out on personalised services. Only 31% of those polled in Britain trust government to use their information compared with 41% who trust the NHS to handle their information. But the media is trusted by just 11%.

Nicholas Watt