He held aloft the traditional red briefcase used to transport the Budget speech along Whitehall to Westminster.

The changes to tax and benefits it contains were revealed in full to the Cabinet on Tuesday morning after the chancellor had breakfast with his Treasury team.

Tony Blair's official spokesman later said the prime minister viewed the package as "radical and imaginative".

Mr Brown has promised to target the family, enterprise and employment in the annual finance bill.

He has previously committed himself to hiking tax on cigarettes and petrol substantially above the rate of inflation.

Budget measures are officially top secret, but speculation has focused on the introduction of a 10p starting rate of income tax.

The chancellor committed himself to this in Labour's election manifesto and may feel the time has now come to implement it.

But Mr Brown's recent speech stressing his commitment to the family has dampened the prospect of a tax on child benefit for the better off.

During his Budget speech, when the nation's eyes are firmly fixed on the Commons despatch box, Mr Brown is expected to follow his own past precedent and sip mineral water.

Other chancellors, including Ken Clarke, have preferred whisky to get them through what can be a gruelling ordeal.

Further back, Benjamin Disraeli drank brandy with water, while his great Victorian rival William Gladstone preferred sherry with a beaten egg.