The new law announced on Saturday is meant to give the public access to government records previously kept secret for 30 years.
Analysts warned the change in legislation is set to pose British Prime Minister Tony Blair a fresh test over Iraq as a newspaper tries to unearth his legal case for war.
And playing down the risk that the government would use veto powers to stop information being published, the Secretary of State for Constitutional Affairs Lord Charles Falconer said such a move would be very exceptional.
"If it is used, the cabinet as a whole has got to agree its use. If a decision were made to use it, then the reason would be given to parliament. It would be susceptible to judicial review," Falconer said.
One British newspaper, The Guardian, says it has asked to see advice to Blair over the legality of the 2003 invasion of Iraq, hoping powers vested in Information Commissioner Richard Thomas will force disclosure.
Thomas could order the information's release on public interest grounds, raising the prospect of a government veto. The paper said the outcome would play a big part in determining public confidence in the new information law.
The long-delayed legislation, which comes into effect decades after similar moves by many nations and a couple of centuries after Sweden, includes a strong public interest clause to override exemptions.
In addition to government, the law also requires regulators, local authorities, publicly owned companies and parliament itself to respond to public requests for information.