When Norman Lamont was Chancellor, the famous red briefcase he carried on Budget day was said to contain a bottle of whiskey. The speech, meanwhile, was carried by an aide. Most tax experts do not expect Gordon Brown’s briefcase to hold anything as exciting, with the Chancellor reluctant to announce measures that may upset his hopes to become prime minister.

Leonie Kerswill, a tax partner at Pricewaterhouse-Coopers, the accountant, says that Mr Brown is almost certain to continue his policy of increasing income tax thresholds only at the rate of inflation. She adds: “Further green measures and progress on tax simplification are likely to be higher on his agenda.”

There is speculation that Mr Brown may offer tax relief for carbon offsetting and reduce VAT on low-energy lightbulbs. There could also be a further increase in fuel duty.

Jonathan Loynes, of Capital Economics, the research consultancy, says: “Fuel duty is a good revenue-raiser, with each 1 per cent increase yielding about £250 million a year.” But with petrol prices still high by historic standards, the political cost of any increase may be too high. “Perhaps a more attractive option would be an increase in road tax,” Mr Loynes adds.

A 20 per cent increase on all road tax bands (taking the annual bill for a small family car from £150 to £180) would raise £1 billion a year. A more politically acceptable option may be to increase the rate only for cars with particularly high emissions.

There is also speculation that the Chancellor may reform stamp duty. Ray Boulger, of John Charcol, the mortgage broker, says: “Rumour has it that he may introduce a 5 per cent top rate for properties worth more than £1 million.”

Ian Luder, of Grant Thornton, the accountant, says that Mr Brown may use extra revenue from any higher band to abolish stamp duty on homes worth less than £250,000.

Few tax experts expect the Chancellor to make any significant increase in the inheritance tax (IHT) threshold, which is already due to rise to £300,000 on April 6. But Peter Twiddy, the recently retired head of the capital taxes office at the Revenue, suggests that Mr Brown is considering a clampdown on IHT planning. He also says that the length of time for financial gifts — also known as potentially exempt transfers — to be free from IHT could increase from seven to ten years.

Tax experts at LexisNexis, the consultant, also believe that IHT relief on AIM-listed company shares may be cut, or even abolished. Jon Golding, of Golding Taxation Services, part of LexisNexis, says: “This area of investment opportunity has now superseded its original intention and is perceived by many as an IHT avoidance ploy for the rich.”

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