Richard Thomas will stress that FoI should not be regarded as a threat - even when it reveals information that is "embarrassing" or "uncomfortable".

And he will caution that "wresting" information from government and other public authorities risks undermining trust between state and citizen.

The public "respects honesty, not cover up", Mr Thomas will say.

His comments, at the annual FoI Live conference in central London, come amid the outcry over a Tory MP's bid to exempt Parliament from its own FoI laws.

Mr Thomas is not expected to refer directly to David Maclean's FoI (Amendment) Bill, but will emphasise the Act "reinforces good government". He will say it is becoming a "fixed feature of 21st century democracy" and should not be viewed as a "battleground".

Mr Thomas will remind public bodies that they serve the public and urge them to adopt a positive approach to openness. But emphasising that FoI must not be used for mischievous and pointless demands, he will announce a "charter" for responsible FoI requests and new guidance for public bodies to resist "vexatious" applications.

"After nearly two-and-a-half years FoI is delivering real benefits," Mr Thomas will say. "There is a presumption of disclosure, unless there is a genuine reason to withhold information. This must trump any instinct of unnecessary secrecy which simply suggests a public authority has something to hide."

And he will argue: "Openness - even where it reveals uncertainties, disagreements or embarrassments - treats citizens as grownups and reflects the realities of public life."

In his speech, Mr Thomas will rebut claims that FoI has an inhibiting effect, insisting that potential disclosure encourages a record of full, accurate and impartial advice from officials. And he will predict: "Greater openness will deter spin and informal decision making."