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as part of what David Cameron says are efforts to lift its "cloak of secrecy".

The first two tranches of data are from 2008/09 and 2009/10.

The Combined Online Information System (Coins) includes what departments were authorised to spend, what they actually spent and what they are forecast to spend in future.

Experts are now poring over the complex files to decipher their contents.

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Mr Rosenbaum, who had a Freedom of Information request to gain access to the database refused last year, said the publication represented a reversal of policy from the previous government.

He said the database had become a symbolic target for open government campaigners - but Labour had argued the information would be impenetrable for most people.

Friday's publication of an "initial tranche" of data for 2008/9 and 2009/10 is part of a series of moves by the government to open up official data to the public.

But as the database contains millions of rows of data, the government admits accessing the files will demand "some degree of technical competence" and they expect expert organisations to make most use of the details intially.

They have asked the Open Knowledge Foundation, an organisation that tries to open up government data, to help make it "more accessible".

They have also promised more details "in more accessible formats" by August.

Open Knowledge Foundation director, Rufus Pollock, said: "The release of this data marks another milestone in the opening up of public data - in which the UK leads the way."

He said the data revealed a "dramatic new level of detail" on how taxpayers' money is spent, of the kind that had previously only been available to civil servants.

For the first time researchers and the public would be able to see how much was being spent in thousands of categories, from pensions to Sure Start and even the Civil List, which decides how much money members of the Royal Family receive.

Previously only the headline figures were available in documents published at the time of the Budget.

He said that it may not lead to "headline-grabbing" stories or "scandals" at this stage, but would "enrich" future research into government spending.

The government said it was "the most detailed UK public expenditure data ever released" and showed it was "committed to being more transparent and open in publishing information about the money it spends".

This week civil servants' salaries above £150,000 were revealed for the first time and hospital infection rates are also due to be published. Information about all future government spending above £25,000 is due to become available from November.

The Coins database contains a detailed analysis of state expenditure under more than 12,000 category headings.

It was at the centre of a political row last year when the then shadow chancellor George Osborne said he was being denied access to information on the database needed to plan future cuts in government spending.

At the time, ministers said that such information was not routinely published, although spending data for the current year, which Mr Osborne wanted, will still not be released under the new regime.

Treasury Chief Secretary Danny Alexander said the Coins release was a "major step forward" - although he promised future releases would be more accessible to the general public.

"For too long the previous government acted as if the public had no right to know where their hard earned taxes were spent. Today we have lifted that veil of secrecy by releasing detailed spending figures dating back to 2008," he said.

"This data is complex, but this is major step forward and shows we are delivering on our promise to make this government more open and transparent while ensuring we deliver value for money for the taxpayer.

"I hope people will take the opportunity to scrutinise carefully how their money is being spent - as I am doing every day in preparation for the spending review."

The move was welcomed by campaign group The Taxpayers Alliance.

Chief executive Matthew Elliott said: "The next step is for this trend to continue, so that all information about government spending is available, allowing the public to call the government to account.

"There is an army of enthusiastic, skilled amateurs out there who will gladly explore and use this information to suggest ways in which the Government can save money and improve public services."