But there was dismay earlier this year when a crucial box of papers relating to the abdication crisis in 1936 was held back by Oxford's Bodleian Library until 2037.

Bemused academics suggested the reason for this could have been because the documents might contain correspondence from the Queen Mother, the publication of which might have proved embarrassing for the Royal Family.

Maximum closure

The government can extend closure on records for several reasons, including the judgement that release could harm international relations or national security.

A mile of shelf space is filled each year

But Elizabeth Honer from the Public Record Office says that one of the main reason documents are given the maximum extended closure is because they relate personal details about people who are still alive.

Disclosure of such information might be a "breach of confidence" or cause "distress" or "endangerment".

"There are some records that perhaps relate to court cases where that material was not made available in open court, particularly if they were rape victims" she says.

"Those sort of things are deemed personally sensitive.

"If that individual were still alive there is no way we would release that information."

Keeping mum

If a government department does want to keep documents secret beyond 30 years, it has to explain why to the PRO.

Filed Away

More than one mile of records are added to PRO each year 2-5% of government records end up at the PRO 900 miles of government records are currently stored Annual cost: �35m to manage and maintain records

According to Ms Honer, it is far from just a rubber-stamping process.

"It is a very rigorous," she said.

"I might say 'You say so-and-so might be personally distressed by this, but where's the evidence?

"Are they still alive? If it is relating to a figure in another country are they still prominent in politics? Are they still in power?'"

An application from a department must go before the Lord Chancellor's Advisory Council on Public Records - this too can also reject applications.

Freedom of information

This year Parliament passed the Freedom of Information Act which creates a statutory right for people to obtain internal documents held by public authorities.

Records span the last 1,000 years

Campaigners have criticised it as being far too weak in comparison to US legislation.

But once it comes in to force, expected to be April 2002, it may begin a significant process of change in Whithall's culture of secrecy.

Records will still be published as usual each January.

But the act allows any determined member of the public to apply for documents held by a public authority without having to wait 30 years.

If the authority refuses to release documents, the applicant will be able to take the case to an information commissioner.

But ultimately, power still rests with the government which will have the final say in the form of a minisiterial veto.

To veto - or not to veto

Maurice Frankel of the Campaign for Freedon of Information said that he believed the Act doesn't go far enough.

"It gives ministers too much discretion," says Mr Frankel.

Elizabeth Honer: Talks tough to government departments

"The great debate is over how inhibited ministers will be about exercising the veto - no-one really knows until it happens."

But Elizabeth Honer says the Freedom of Information Act will have a significant effect on Whitehall.

"The fundamental difference is a conceptual one. At the moment with the 30-year rule there is this view that record should remain secret for 30 years. FOI turns that on its head.

"The assumption is no longer that they are closed for 30 years but that they are open from the point of creation, unless there is a justification for keeping them closed.

"Culturally that is a big shift, and I think it's a healthy shift."

Mr Frankel also welcomes the change but describes the FOI as a "half-way house".

"At the end of 30 years the government publishes a whole year's worth of papers.

"Under FOI you've got to be able to pinpoint what you are interested in - but you don't have to wait 30 years.

"And that's an enormous advantage."