Shell is coming under intense pressure to scrap a £5 billion natural gas project in Iran after a call from a group of leading US pension funds for energy companies around the world to cut their ties with the Middle Eastern state.

Some of America’s most influential institutional investors have written to Shell and seven of its major rivals, including Russia’s Gazprom and Total, warning that there is an increasing risk that sanctions imposed on the country will hit their business.

In a two-page letter sent to Jeroen van der Veer, Shell’s chief executive, the funds have revealed they are even concerned that infrastructure developed by the company could be targeted in any future war.

It stated: “Historically, regional conflict has exposed facilities and infrastructure to the risk of attack, as evidenced by the Iran-Iraq war of the 1980s.”

The move threatens to overshadow what promises to be one of the best set of quarterly results from Shell for nine months today. Analysts expected the group to reveal second quarter profits of $6.7 billion (£3.2 billion).

The concerned shareholders include New York City’s five main pension funds and the influential California Public Employees Retirement System (Calpers), which has $700 million invested in Shell.

The funds are coming under increasing pressure from State legislators in the US to divest their holdings in companies with ties to Iran.

American oil and gas companies have already been barred from dealing with Iran, a stance backed by Lord Browne of Madingley, the former chief executive of BP, two years ago.He said: “Politically, Iran is not a flyer.” Shell has a long history with the Middle Eastern state, and three years ago signed a framework agreement to develop part of the giant South Pars gasfield and a liquefied natural gas plant with Spain’s Repsol.

Mr van der Veer has continually refused to clarify Shell’s position regarding Iran, insisting that a final investment decision is more than a year away.

A spokesman last night refused to comment on the letter except to confirm it had been received. He said: “When we come to deciding on Iran, we will take political considerations into account.”

Iran holds the second-biggest gas reserves in the world behind Russia and is seen by analysts as a key part of Shell’s portfolio, given the company’s need to replenish its oil and gas developments.

Other companies on the pension funds’ hit list include Repsol, Italy’s ENI, China National Petroleum Corp, ONGC of India and Japan’s Inpex Corporation. In total they hold $3.7 billion worth of shares in energy companies involved in Iran.

Industry sources last night told The Times that pressure from the US is already making it harder for British exporters to bank letters of credit from Tehran. UK companies sell hundreds of products for oil pipelines in Iran.

A spokesman for Calpers insisted that the letter sent to Shell and its rivals was little more than a “fact-find-ing mission” to establish the extent of their operations in the country.

However, he admitted there was increasing concern about the companies’ ties to a “state sponsor of terrorism”. “It is increasingly likely that the worsening situation will negatively impact companies doing business there,” he said.

Iran’s pool

— Iran has 132.5 billion barrels of proven oil reserves – 10 per cent of the world’s total

— It holds 970 trillion cubic feet of natural gas, second only to Russia

— The country is the second-biggest Opec member behind Saudi Arabia, producing nearly four million barrels of oil a day, more than either BP or Shell’s global daily output

— Oil exports generate more than $45 billion a year for Iran, half its annual budget

— Exports are up by 40 per cent compared with the level just three years ago

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