The UK tax body says it has reduced the tax gap but claims that the largest amount of unpaid tax is owed by the nation's small firms.
The tax gap is the difference between the tax that should be paid to HM Revenue and Customs (HMRC) and the actual tax that has been paid. HMRC's latest figures show that the tax gap has fallen from 7.3% in 2005/06 to an estimated 5.7% in 2016/17 - worth £71 billion in tax revenues.
HMRC says it has been focusing its efforts on getting the tax gap down by tackling tax evasion and avoidance and also by helping tax-payers to get their tax return information right.
According to HMRC, small firms are responsible for 41% of the current tax gap, followed by large businesses at 21% and mid-sized firms at 12%. Criminals are responsible for 16% of the gap.
Brian Palmer, tax policy adviser at the Association of Accounting Technicians (AAT), said: "The public might be surprised on first glance that SMEs are the biggest contributors to the tax gap. But, of course, large firms have the professional and financial expertise to ensure they are correctly paying the taxes they owe - not a penny less, crucially, but also not a penny more."
Mel Stride, financial secretary to the Treasury, said: "These really positive figures show that the tax gap is the lowest in the last five years, which reflects the hard work that HMRC and I have been doing to ensure we support businesses to pay the right tax at the right time and clamp down on tax evasion and avoidance."
Jon Thompson, HMRC's chief executive, said: "The UK is the only country in the world to regularly publish their tax gap in detail and at 5.7%, it remains at its lowest for five years. I am pleased that the downward trend shows HMRC and HM Treasury's continued hard work to tackle evasion and avoidance is working."
HMRC said it is working with small businesses to help them get their tax right first time around and that it aims to make sure the tax system is not a barrier to setting up, running and growing a business. Digital record keeping and an automated tax system - with the roll-out of Making Tax Digital - will help businesses get their tax affairs right the first time, says HMRC.