The British broadsheet says it obtained confidential documents revealing that Shell paid Iran at least $1.5 billion for crude oil in the summer months alone, as competitors including BP halted orders from the Islamic Republic.
Iran is under strict economic and military embargoes implemented by the United Nations Security Council and many countries unilaterally, including, of course, the United States.
Shell's only response to the Guardian's report was to insist that the purchase of Iranian crude in no way violates any sanctions.
The importation of oil from Iran has not been banned by the sanctions currently in place, but most large petroleum companies have voluntarily stopped purchasing from the nation.
Any money paid to Iran for crude goes to the state-owned oil company, and in turn to help fund the government. Run by Islamic clerics and hardline President Mahmoud Ahmadinejad, the Islamic government has made the development of its nuclear power program a cornerstone policy.
The U.S. government and many of its allies insist the energy program is hiding a clandestine effort by the Iranians to develop nuclear weapons capability - a charge the country's leaders flatly deny.
British foreign secretary William Hague said recently that the U.K. does not encourage trade with Iran due to "serious concerns about the nature of Iran's nuclear program," reports the Guardian.
Asked to comment on the story Monday night, a spokesman for Hague's office told the newspaper that the government was, "serious about intensifying the pressure on Iran to return to the negotiating table."