The world's biggest oil company, Exxon Mobil, has softened its hardline position on climate change by throwing its weight behind a tax on carbon emissions.

In a significant shift in stance, Exxon's chief executive, Rex Tillerson, told an audience in Washington that he considered a tax to be a fairer route to curbing emissions than a cap-and-trade system of pollution allocations.

"As a businessman it is hard to speak favourably about any new tax," said Tillerson. "But a carbon tax strikes me as a more direct, a more transparent and a more effective approach."

Until recently, Exxon was reluctant even to concede that greenhouse gas emissions were responsible for global warming. The company has faced mounting pressure over its environmental policies, culminating in a shareholder rebellion at its annual meeting last year led by members of the oil-rich Rockefeller family.

In his speech, Tillerson suggested that a combination of greater efficiency, new sources of energy and curbs in demand were needed in a "multidimensional approach" to meeting challenges in policy.

"A carbon tax is also the most efficient means of reflecting the cost of carbon in all economic decisions - from investments made by companies to fuel their requirements to the product choices made by consumers," he said.

The US's second-largest oil firm, Chevron, warned that its profits would be lower because of a slump in the price of oil. The cost of a barrel of crude dipped below $40 yesterday, down from its peak of $147 in July.