Business Secretary Vince Cable has been fined for failing to pay up to £25,000 in VAT on his media work, it has been reported.
According to the Sun, Mr Cable was ordered to pay a £500 penalty by HM Revenue and Customs after it emerged he had not paid the tax on time.
His aides told the newspaper the mistake was made "unknowingly".
In the two years before he became minister, Mr Cable is said to have made a considerable sum from media work and book deals, although he cut back his freelance work after joining the Government in May 2010.
He earned an estimated £192,000 on top of his MP's annual salary of £65,738, but did not tell HMRC that his income had exceeded a pre-set threshold, it was reported.
HMRC rules state that if a person's turnover of VAT-liable goods and services over a 12-month period exceeds the current threshold of £73,000, they must register for the duty within 30 days.
Mr Cable's aides told the newspaper the error was spotted by his personal accountants in January while his tax return for the previous financial year was being finalised. The accountants notified HMRC officials and apologised, before offering to pay any due tax.
The former deputy leader of the Liberal Democrats was then billed for a figure of "less than £15,000" after a rebate on his charity donations was taken into account, his spokeswoman told the Sun.
He is said to have sent a cheque to the Revenue by mid-February to make up for the shortfall, and was let off with half the usual penalty of £1,000.
Mr Cable's spokeswoman said in a statement: "All Vince's tax affairs are above board, and he went out of his way to settle this quickly - in fact HMRC let him off 50% of the usual penalty."