The news comes a day after The Times revealed that another critical project in Siberia had spiralled more than $2 billion over budget in just two months.

The troubled Anglo-Dutch giant said that production at the Bonga installation, potentially the largest oilfield in Nigeria with an estimated output of more than 225,000 barrels of crude a day, would not start until 2005 at the earliest.

In February, Shell put back the start-up date for the site by about six months to “late 2004”, citing operational problems at the development,which is located in coastal waters more than 1,000m deep.

The company also admitted that it was “facing cost pressures” on the $10 billion (£5.4 billion) Sakhalin-2 Siberian natural gas project, adding that it was reviewing both the “cost and execution stages” of the development.

The statement follows disclosures in The Times that the total cost of building Russia’s largest energy project was now more than 20 per cent higher than the budgets presented to analysts in February.

Shell conceded yesterday that the Sakhalin-2 project had been hit by “movements in foreign exchange rates” and “growth in the regional economy”, which has pushed up the costs of raw materials in Eastern Siberia. A spokesman said that the review was “normal practice in large, complex projects”.

Company officials hinted that an official statement to the Stock Exchange would be necessary on Sakhalin-2, if the review concludes that the cost overruns are “material”.

Investors were last updated on the status of the Bonga and Sakhalin-2 projects in February, when Sir Philip Watts, chairman at the time, was fighting to save his career after an admission that the company had been lining its balance sheet with 3.9 billion barrels of unproven reserves.

Sir Philip has since been ousted, and the oil giant, once famed for its conservatism, is now the subject of investigations by both the US Securities and Exchange Commission, the market regulator, and the US Department of Justice.

Regarding the Nigerian Bonga project, a Shell spokesman said: “As we indicated in February, we had some challenges to deal with on Bonga — we have had to absorb additional offshore workscope that has delayed the project.

“We’re working to get it up and running as quickly as possible, and we expect it to be on stream next year.”

Bonga was first announced in December 1999, by Sir Philip, when he was running the group’s exploration and production division. The project aims to extract an estimated 600 million barrels of oil that are lying 120 km off the coast of Nigeria. Shell has a 55 per cent stake in the venture, with the balance being held by Exxon Mobil, Eni and Total.

The Bonga project was originally set to come on stream in 2003. At that time, Sir Philip said that the project was a “technological triumph that we are able economically to produce oil and gas from such deep water, and in such a compressed timescale”.

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