Releasing sensitive Government documents 10 years earlier than current rules allow will cost nearly £40m, ministers have said.
Around 3.3 million records, including the minutes of Cabinet meetings, will be affected by the move to a ‘20-year rule’ from a 30-year ban on publication.
The change will be phased in over 10 years by doubling the old records released each year to the National Archive from 2013.
The transition period is expected to cost between £34.7m and £38.5m, according to the Ministry of Justice.
It comes after a review by Daily Mail editor Paul Dacre under the last Government which recommended shortening the time official documents remain secret.
Lord McNally, the minister responsible for archives, said: “The change to a 20-year rule is a key part of our transparency agenda and will see a wealth of historical material opened to the public much earlier.
“The aim is to provide greater openness and accountability, strengthening democracy through more timely public scrutiny of government policy and decision-making.”