Thousands more middle-income earners could be dragged into the 40% tax net if the government freezes allowances in this week's pre-budget report, as is widely expected.
Another 70,000 people would pay the 40% rate, raising about £280m in extra income tax for the government in the 2010 tax year, according to figures from Grant Thornton, the accountant.
Alistair Darling, the chancellor, is expected to keep the personal allowance - the amount of income on which no tax is paid - at £6,475, with the point at which you start paying higher-rate tax frozen at £43,875. He will argue that this is justified because thresholds are linked to inflation on the retail prices index, which fell 1.4% in the year to September.
However, accountants argue that freezing the threshold would constitute a stealth tax because earnings rose at an average of 1.2% in the year to September, according to the Office for National Statistics. If earnings rise while thresholds are frozen, more of your income becomes subject to higher-rate tax. The move would mean someone earning £43,500 this year - a basic-rate taxpayer - would be bumped into the 40% rate next year if their wages rise in line with average earnings.
The public sector is likely to be hit the hardest because earnings are up 2.8% in the year to September, compared with just 0.8% in the private sector. Many key workers, such as nurses and police officers, earn around the £43,500 mark.
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Mike Warburton of Grant Thornton, the accountant, said: "These people will fork out a higher portion of their income in higher-rate tax if thresholds are frozen. Personal allowances should be linked to earnings, not inflation. It would be the fair thing to do to increase personal allowances at 1.2%."
Labour froze personal allowances in the 2002 budget, even though inflation had risen, in a £700m tax grab. Darling is also expected to freeze the inheritance tax threshold, which was due to go up above inflation from £325,000 to £350,000 in April, or £650,000 to £700,000 for couples. However, accountants said this could backfire on the government, as IHT is highly unpopular with voters.
The chancellor could also use Wednesday's announcement to launch an attack on tax avoidance schemes that favour the wealthy, to quell growing public anger against bankers' bonuses.
Accountants expect the chancellor to flag laws to eradicate schemes popular with banks trying to shield bonuses from tax, such as employee benefit trusts and employer-financed retirement benefit schemes.
Tax advisers have persisted in setting up such schemes for firms, costing the Treasury tens of millions of pounds, despite warnings from HM Revenue & Customs, UHY Hacker Young, the accountant, said.
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Employers who bring forward bonus payments to avoid the clampdown could also be targeted this week.
Accountants have reported that the increase in the top rate of tax on incomes over £150,000 - due to come in on April 6 - is prompting employers to consider bringing forward bonus payments so that employees pay 40% tax instead.
However, if the bonus payment pushes your salary over £150,000, you will be caught out by pensions changes announced in May's budget. Stephen Green of Watson Wyatt, the actuary, said: "Having gone over £150,000 in this tax year, you would be subject to a 20% charge on any higher pension saving you might have planned in the next tax year, even if your income falls back below £150,000."
STEALTH MOVES EXPLAINED
How much more inheritance tax could I pay?
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Your heirs would end up paying 40% IHT on the extra £25,000 that is subject to tax - that is £10,000 per individual.
How much will the frozen personal tax allowance cost me?
Someone earning £43,500 this year and now paying £7,405 in income tax will pay £134 a year more if their earnings rise in line with the national average.
Which schemes face a clampdown?
Employee benefit trusts (EBTs) can be used to avoid tax and National Insurance contributions on bonus payments. The schemes involve companies setting up trusts in the name of the employee or their family.
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Say a higher-rate taxpayer is paid a £100,000 bonus through the tax system, on which they would pay tax of £50,000 from next April. They could put £100,000 into a trust. There is no tax on payment into the trust and the payment usually rolls up offshore tax-free.The trust then makes a loan of £100,000 to the employee, which can be bought back into the UK. Tax would be paid on interest payments at a notional 6% - 40% on £6,000, or just £2,400. The loan would then be written off.
David Kilshaw of KMPG said: "HMRC doesn't like EBTs so changes are possible, such as imposing a tax charge when the monies are paid into the trust."