Nearly one in five people who have completed a tax return think they may have made a mistake that has potentially cost them money, a survey has found.
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Which? found 19% of people who had filled in a self-assessment tax return in the past two years believe they may have lost out financially because they had made an error or not understood the document.
People were most likely to say they struggled to understand tax rules surrounding property income, while some also said they struggled to understand rules around dividend income, pension contributions or rates and allowances for savings income, the research found.
The deadline for sending 2015-16 self-assessment tax returns online to HM Revenue and Customs (HMRC), and paying any tax owed, is January 31.
Which? has a new online tax calculator, developed with SimpleTax, - at which.co.uk/taxcalculator - to help people complete their tax returns.
The calculator is free for Which? members, including those on a membership trial, and enables them to calculate their tax bill and submit it directly to HMRC, while non-members would need to pay £18 to do this.
More than 600 people who have filled in a self-assessment tax return in the past two years took part in the survey.
Penalties for late tax returns include an initial £100 fixed penalty, which applies even if there is no tax to pay, or if the tax due is paid on time.
HMRC recently released a list of bizarre excuses for people not getting in their returns on time, which were used in unsuccessful appeals against its penalties for late returns.
They included a return being on a yacht that caught fire and a dog eating a return and all the reminders.
HMRC has previously said it will treat those with genuine excuses for late returns leniently and focus its penalties on those who persistently fail to complete their tax returns and deliberate tax-evaders.
An HMRC spokesman said: "Taxpayers can amend their returns within 12 months of the original deadline. There is lots of help available online. Anyone who needs help should get in touch with us."