The biggest ever sale of oil assets will take place today, when the Iraqi government puts 40bn barrels of recoverable reserves up for offer in London.

BP, Shell and ExxonMobil are all expected to attend a meeting at the Park Lane Hotel in Mayfair with the Iraqi oil minister, Hussein al-Shahristani.

Access is being given to eight fields, representing about 40% of the Middle Eastern nation's reserves, at a time when the country remains under occupation by US and British forces.

Two smaller agreements have already been signed with Shell and the China National Petroleum Corporation, but today's sale will ignite arguments over whether the overthrow of Saddam Hussein was a "war for oil" that is now to be consummated by western multinationals seizing control of strategic Iraqi reserves.

Al-Shahristani is expected to reveal some kind of "risk service agreements" that could run for up to 20 years, with formal offers to be submitted by next spring and agreements signed in the summer.

Gregg Muttitt, from the UK-based social and ecological justice group Platform, says he is alarmed that the government is pushing ahead with its plans without the support of many in Iraq.

"Most of the terms of what is being offered have not been disclosed. There are security, political and reputational risks here for oil companies but none of them will want to see one of their competitors gain an advantage," he said.

Heinrich Matthee, a senior Middle East analyst at the specialist risk consultant Control Risks Group, also believes there are many pitfalls for those considering whether to make an offer.

"Currently it is unclear which party in Iraq is authorised to award a contract and at the same time to deliver its side of the bargain," he said. "Any contract with an independent oil company will be subjected to opposition and possible revision after pressure by resource nationalists."

Oil companies will find their reputations at risk from the actions of their Iraqi counterparties, such as joint venture partners, suppliers and agents. They will also have to contend with oil smuggling and the possibility that the ruling alliance could collapse, Matthee said.

He said that if the conspiracy theory that western oil companies egged on US and British governments to invade Iraq were true, the plan could backfire on them and benefit rivals in Asia instead. "It is possible the American army has provided the economic stability that will encourage Malaysian, Chinese and other Asian companies to become involved," he said.

There is no precedent for proven oil reserves of this magnitude being offered up for sale, said Muttitt. "The nearest thing would be the post-Soviet sale of the Kashagan field [in the Caspian Sea], which had 7bn or 8bn barrels."

China's state-owned oil group, CNPC, has already agreed a $3bn (£1.78bn) oil services contract with the government of Iraq to pump oil from the Ahdab oil field.

The deal is the first major oil contract with a foreign firm since the US-led war and was followed up by an agreement with Shell, potentially worth $4bn, to develop a joint venture with the South Gas Company in Basra.

This deal has also triggered controversy. Issam al-Chalabi, Iraq's oil minister between 1987 and 1990, questioned why there had been no competitive tendering for the gas-gathering contract and claimed it had gone to Shell as the spoils of war.

"Why choose Shell when you could have chosen ExxonMobil, Chevron, BG or Gazprom?" he asked. "Shell appears to be paying $4bn to get hold of assets that in 20 years could be worth $40bn. Iraq is giving away half its gas wealth and yet this work could have been done by Iraq itself."

The Baghdad government says it aims to increase crude oil production from 2.5m barrels a day to 4.5m by 2013, but faces internal opposition from regional governors and political opponents.

The sale today comes as oil prices have plummeted after stockmarket turmoil on Friday. The price of crude fell by more than $4 at one point to $75 a barrel - the lowest point since September last year and a sharp drop from its peak of $147 in July. Opec, the oil producers' cartel, has called an emergency meeting to agree a cut in output to bolster prices in spite of protestations from politicians including Gordon Brown. Brown said on Friday: "We've had some success in getting the price of oil down: the price this morning is roughly $80, about half what it was a few months ago. I want these price cuts passed on to the consumer as quickly as possible.

"I'm concerned when I hear that the Opec countries are meeting, or are about to meet, to discuss cutting production - in other words, making the price potentially higher than it should be.

"I'm making it clear to Opec it would be wrong for the world economy and wrong for British people who are paying high petrol prices and high fuel prices to cut production and therefore keep prices high."

A government source said: "The one chink of light has been the fall in the price of oil. The last thing we want is to head into a difficult period with a return to high oil prices. People need to act responsibly."