The move comes as changes in allowances for married couples and single parents announced in last November's Budget may have left thousands of people with the wrong tax code.
Help the Aged has also estimated that an additional 150,000 single pensioners will now be paying tax, after the Chancellor froze personal allowances at last year's levels.
BDO Binder Hamlyn operates a sliding scale of charges, depending on the complexity of cases. A typical annual fee for a retired couple, with a company pension and pounds 5,000 of dividends from investments and a capital gains tax liability, would be pounds 325 plus VAT.
A single person with company car and various fringe benefits, plus shares from a number of privatisation issues, would expect to pay pounds 450 plus VAT for the service.
MacIntyre Hudson has launched a similar scheme, called ActionTax. The company's fees range between pounds 120 for a client living outside London to pounds 650 for someone in London.
Michael Hayes, a tax partner at MacIntyre Hudson, said: 'We have introduced our service to help taxpayers by relieving them from the time and trouble needed to complete their returns. We may also be able to save money for many of them, who are paying too much tax.'
Mr Hayes said the system would be particularly useful once self-assessment for personal income tax is introduced in 1997.
'There is a genuine concern that people with a background in financial services, or from banks and building societies, could just as easily provide a similar service by using increasingly sophisticated software.
'We could end up with a situation where the salesman from a life company sells you the policy and then offers to sort out your tax affairs.'
In Coventry, a firm of accountants called the Tax Company is charging a pounds 50 fixed fee for each return, regardless of size or complexity.
Tax assessments and coding notices are checked for just pounds 6. The company claims it is so cheap because of its advanced technology.
Help the Aged is offering a free 16-page pamphlet which explains what part of a person's income is taxable, what allowances are available and how much should be paid.
Help the Aged pamphlet from Information Department, Help the Aged, St James's Walk, London, EC1 0BE. MacIntyre Hudson ActionTax: 0908 609999. The Tax Company: 0203 552077.