When someone intentionally understates their income on their tax return for financial gain, this is known as tax fraud or tax evasion. To commit tax fraud, they must do so intentionally and on purpose. Tax fraud is an extremely serious offence in the United Kingdom. As a result, the consequences of tax fraud can be severe. If you are convicted of tax fraud, you might face a maximum sentence of 7 years in prison, as well as a significant fine and other financial penalties. You may also be required to return what you owe.