Company says it 'ceased transactions prior to EU sanctions' when it learned of links with Atomic Energy Organisation of Iran

One of Britain's biggest companies has made millions of pounds selling goods to Iran, including to a state-owned firm that supplies the regime's nuclear programme.

Glencore, a commodity trading house run by the billionaire Ivan Glasenberg, traded $659m (£430m) of goods, including aluminium oxide, to Iran last year, the Guardian has established.

The company, which is one of the biggest businesses in the FTSE 100 and has a market value more than three times that of Marks & Spencer, has admitted that some of its aluminium oxide ended up in the hands of Iranian Aluminium Company (Iralco).

Trafigura, another commodity trading house, has also admitted to trading an unspecified aluminium oxide (also known as alumina) with Iralco in the past.

The International Atomic Energy Agency has named Iralco as supplying aluminium to Iran Centrifuge Technology Company (Tesa), which is part of the Atomic Energy Organisation of Iran (AEOI). Aluminium oxide is an important material in gas centrifuges used to enrich uranium.

At the time of the Glencore and Trafigura trades with Iralco, it was not illegal or a breach of sanctions to supply Iran with alumina. It is unknown whether Glencore or Trafigura's alumina passed from Iralco to Tesa, or whether it was used in centrifuge construction.

Since 2006, AEOI has been subject to UN sanctions designed to prevent a drive by Iran toward nuclear armament. Trading with Tesa has been specifically banned under US, EU and UK sanctions since July 2010. Iralco was added to the EU sanctions list in December 2012.

Glencore said it "ceased transactions" with Iralco immediately when it learned of its links with Tesa, and the last trade was in October 2012. "Prior to EU sanctions in December 2012, we were not aware of a link/contract between Iralco and Tesa," the company said in a statement.

Glencore said it is "reliant on the relevant regulatory bodies/governments to advise us on developments in who we can/can't do business with".

Tehran, which some experts say already has enough enriched uranium to make several nuclear weapons, is in the middle of upgrading its stock of more than 10,000 centrifuges. The IAEA said Iran is replacing outdated centrifuges with thousands of more powerful IR-2m models.

Experts at the Institute for Science and International Security (Isis) in London said: "Iran is trying to replace maraging [super-strong] steel end-caps with high strength aluminium end-caps."

Mark Fitzpatrick, director of Isis's nonproliferation and disarmament programme, said the new centrifuges could enrich uranium four to five times faster than the existing ones. Iran insists its enriched material is for peaceful use, not for nuclear weapons, but it has refused to allow IAEA inspectors into several of its atomic facilities.

The question surrounding Glencore's role in unintentionally potentially helping arm a nuclear Iran comes as Obama ramps up pressure on Tehran. This month, the US secretary of state, John Kerry, said: "We understand the nature of the threat of Iran. And as the president has said many times – he doesn't bluff. He is serious. We will stand with Israel against this threat and with the rest of the world, who have underscored that all we are looking for is Iran to live up to its international obligations."

Mark Wallace, a former US ambassador to the UN, said Glencore's dealings with Iran were "completely unacceptable", adding: "We might expect this from a Russian or Chinese company, but the truth is that even those companies usually stay away from this sort of exposure."

Glencore said it "complies with applicable laws and regulations, including applicable sanctions. We closely monitor all new legal developments to ensure that we continue to be in compliance with applicable laws and regulations, including applicable sanctions."

Details of the firm's dealings with Iralco were leaked to the media in February, but the company declined to specify how much the deals were worth.

The Guardian has learned that Glencore traded $659m worth of metals, wheat and coal with Iranian entities during 2012. Buried deep in its annual report, one of Glencore's US affiliates, Century Aluminium, 46% owned by Glencore, states: "During 2012 non-US affiliates of the largest stockholder of the company [Glencore] entered into sales contracts for wheat and coal as well as sale and purchase contracts for metal oxides and metals with Iranian entities, which are either fully or majority owned by the GOI [government of Iran]."

Glencore declined to state how much of the $659m it dealt with Iran in 2012 was related to alumina/aluminium. The trades were not illegal or against sanctions at the time. It is not the first time Glencore's activities have attracted controversy. Last year the head of its food trading business said the worst drought to hit the US since the 1930s would be "good for Glencore" because it would lead to opportunities to exploit soaring prices. It has also attracted attention by selling more than £50m worth of wheat to the World Food Programme.

Trafigura, which came to global political attention when it was revealed that a licensed independent contractor of a ship it had chartered dumped tonnes of toxic oil slops in Ivory Coast, said: "We can confirm that Trafigura has traded with Iralco in the past. In October 2011, a physical swap agreement was reached whereby Trafigura provided alumina to Iralco in return for aluminium for Trafigura to export worldwide. No deliveries have been made or exports received since new EU sanctions were published in December 2012. Trafigura Group companies are compliant with national and international law where applicable."

• The headline and text on this article were amended on 24 April 2013 to refer to Iran's nuclear programme, rather than its nuclear weapons programme.