Shadow chancellor John McDonnell has written to Chancellor George Osborne demanding details of the £130 million tax settlement with Google.
Last week, HM Revenue & Customs (HMRC) reached an agreement with the global tech giant to backdate its taxes, having paid just £70m on profits of £7.2 billion since 2005. Critics labelled the settlement as "derisory", stressing that most businesses paid corporation tax of 20% - which would make its total bill for the period roughly £1.6bn. However, the settlement amounts to around 3%.
The shadow chancellor has asked Mr Osborne if he himself, or his advisers, were involved in the settlement in any way. He has specifically questioned the Chancellor on when he became aware of the agreement, and whether or not he, or a ministerial colleague signed off on the deal.
Mr McDonnell asked: "What is HMRC's understanding of the effective tax rate faced by Google over the past 10 years as a result of this settlement?"
"I know that many are concerned about the tax treatment of large companies and it is important for public trust that HMRC is fair and transparent in its dealings with such companies."
The government and HMRC have been forced to defend the deal, with Mr Osborne describing it as "a major success of our tax policy". HMRC said the settlement meant Google paid "full tax due in law".
Speaking to the BBC, Jim Harra, HMRC's head of business tax: "We only accept the full amount of tax, interest and penalties that is due, otherwise if we can't reach an agreement on that amount we will go to tribunal. We certainly don't apply any rate of tax other than the statutory rate that Parliament has published."