The editor of The Guardian newspaper is to be grilled by MPs today over the publication of information contained in top-secret documents leaked by US whistleblower Edward Snowden.
Alan Rusbridger, who has been editor of The Guardian since 1995, will face questions from the Home Affairs Select Committee.
Mr Rusbridger, 59, and The Guardian have faced criticism for publishing details of the activities of the UK's listening post GCHQ and its US counterpart the National Security Agency (NSA).
The revelations were disclosed by former NSA contractor Mr Snowden to The Guardian and other newspapers worldwide including The Washington Post and Germany's Der Spiegel.
Critics, including the heads of GCHQ, MI5 and MI6, have claimed Mr Rusbridger's decision to publish has aided terrorists, while others believe the move may be illegal.
However, champions of the newspaper have hailed its positive influence on public debate around the reach and powers of the security services and intelligence agencies.
The newspaper was named Independent Voice of the Year at human rights campaigners Liberty's annual awards ceremony last month.
Shami Chakrabarti, director of Liberty, said: "Whatever your position on blanket surveillance, the people and their parliament have a right to debate it and there can be no debate about what we do not know."
Earlier this year, MI5 director general Andrew Parker warned in a speech that revealing details about the work of GCHQ was a "gift to terrorists".
And later, in an unprecedented move, the three heads of MI5, MI6 and GCHQ appeared together in public for the first time before the Intelligence Security Committee to discuss the work of their respective agencies.
Sir John Sawers, head of MI6, said terrorists were "rubbing their hands with glee" at the Snowden disclosures, while GCHQ chief Sir Iain Lobban claimed Britain's enemies had learned how to avoid vulnerable communication methods.
Among the revelations contained within the NSA files leaked by Mr Snowden was details of mass surveillance programmes such as the NSA-run Prism and the GCHQ-operated Tempora.
Under the £1 billion Tempora operation, Cheltenham-based GCHQ is understood to have secretly accessed fibre-optic cables carrying huge amounts of internet and communications data and shared the information with the NSA.
It was also claimed that intelligence agents track global leaders' phone calls after being given their numbers by an official in another US government department, leaked documents reveal.
The revelation added to diplomatic tensions between the US and its allies, after it was claimed that the phone of German Chancellor Angela Merkel was tapped for 10 years.
Mr Rusbridger will be followed by the Metropolitan Police Commissioner Sir Bernard Hogan-Howe and Cressida Dick, Assistant Commissioner with the Metropolitan Police.