HM Revenue & Customs is penalising oversights and mistakes in tax forms and sending out blanket warning letters to people who have filled in their forms correctly, according to Baker Tilly.
Mike Down, the accountancy firm's head of tax risk and investigations, said there were now "worrying trends" in HMRC's approach. "Innocent mistakes were never intended to be brought within the new penalty regime when it was introduced six years ago."
Mr Down went on: "HMRC is keen to treat all tax errors as potentially liable to penalties, and now consider the starting point for taxpayer behaviour as 'deliberate'. All this is thrown into sharper focus when you consider HMRC's proposals to access taxpayers' bank accounts." He said the department might promise safeguards but had "a tendency to forget that the taxpayer is a person, and not merely an object of cash generation".
Baker Tilly has also accused HMRC of "issuing a string of blanket letters to unsuspecting - and now worried and confused - customers asking them to check that their self-assessment returns are in order", when simple checks would have shown why someone's tax rate was lower than might be expected.
A spokesman for HMRC said: "We're committed to making it easy for our customers to get their tax right. We are issuing letters as part of a trial to help individuals identify any mistakes they may have made on their self assessment return. If a customer is content that their return is accurate then they do not need to do anything. Anyone who needs help is welcome to get in touch with us."
Ronnie Ludwig, at accountants Saffery Champness in Edinburgh, said recent celebrity cases should not obscure taxpayers' rights. "Tax avoidance isn't illegal, and everyone is keen to avoid paying unnecessary gratuitous levies to the government if it can be avoided.
"Simple steps like income and asset splitting with spouses, making lifetime gifts to adult children and making sure that interest on borrowings will qualify for tax relief, can all make a substantial difference without upsetting HMRC."
The row came as think tank the Adam Smith Institute declared May 28 as this year's 'Tax Freedom Day', the day on which the average UK citizen starts pocketing their earnings, after nearly five months of working entirely to fund the combined costs of the state, through a combination of income tax, national insurance, VAT, fuel duty, council tax, alcohol and tobacco duty and other levies.
For higher earners, the day they start working for themselves could be later, said Jason Hollands at Bestinvest, which next month becomes Tilney Bestinvest with 46 staff in Scotland. Mr Hollands said: "While the Coalition has raised the annual personal allowance, taking large numbers of lower income voters out of income tax altogether, the Institute for Fiscal Studies estimates that by 2015, around two million more people will have been drawn into paying tax at 40 per cent since the start of the Coalition."
He went on: "Estimating just how much income tax we individually pay in the UK, let alone all the other taxes, is not an easy task." Bestinvest has a calculator to help any taxpayer work out their net income after income tax, national insurance, pension contributions and student loans.
Meanwhile HMRC this week began sending tax reconciliations to millions of employed taxpayers and pensioners for 2013/14.
Many of these will show that there is tax outstanding and demand payment.
Anthony Thomas, chairman of the Low Incomes Tax Reform Group, commented: "Millions of these reconciliations are issued each year and while most of them are correct, many have discrepancies that can be due to mistakes made by the employer or pension provider, or HMRC. Sometimes when that happens the taxpayer will be able to have the tax due written off or reduced, but this can take considerable time to sort out."
The LITRG has a guide to help taxpayers understand their liability.
Mr Thomas said: "What is crucial is that you do check it - sometimes there are mistakes and sometimes there are extra reliefs that you can claim." Payments should not be made pending resolution, and dates and times of phone calls to HMRC should be noted, Mr Thomas said.
The average retired household pays out 30 per cent of its annual income in a combination of direct and indirect taxes, according to new analysis of government data by Prudential.
Stan Russell, retirement income expert at Prudential, said: "Retiring from work doesn't mean that you are retiring from paying tax."
VAT and income tax each consume 8 per cent of the average retired household's annual income and council tax 4 per cent, while up to 10 per cent can go in vehicle excise duty and taxes on fuel, alcohol and tobacco.