The chancellor has welcomed commitments from oil producing countries to increase production but warned them that they must do more.

Gordon Brown said on Thursday that the move by the Opec group was a "welcome and necessary first step" as petrol prices rose above £1 per litre in some areas.

He pledged to continue putting pressure on the Middle Eastern cartel after the Opec meeting was unexpectedly vague in its outcome.

Brown told the BBC that "more can be done and more will be done" by Opec to bring prices down following instability in Saudi Arabia and high demand from China and the US. "Production has got to continue to rise," he said.

The Treasury chief also left the door open a rethink on his planned increase in fuel duty which is set to kick-in in September.

The chancellor has been facing calls from the opposition and motorists groups to postpone or scrap the tax hike altogether.

Giving the first hint that he might make a rare shift in policy, he also insisted that it was "inadequate" production that was behind the recent price rises rather than his tax regime.

"I will review the situation again in August," he said.

But Brown added that it would be "opportunistic and short term to move the focus away from the source of the problem.

At a press conference in Downing Street the prime minister said the government would assess the impact of the Opec move first.

"We have got to see how that settles down," Tony Blair told reporters.

"Of course we are sympathetic to the needs of businesses and motorists," he added.

"We don't need to take a decision right now but we will take it later."

Campaigners

As the debate over rising petrol prices continued, environmental campaigners have urged the chancellor not to back down on his plans to raise the level of petrol duty.

In an open letter to Brown, key campaign groups argued that the environmental costs of motoring justified plans to increase the tax.

Leaders of Transport 2000, the Green Alliance, Friends of the Earth, Greenpeace UK, WWF-UK and the RSPB said the calls of protesters should be ignored.

"We believe that the focus by protesters on the 1.9p duty increase is a distraction from the bigger environmental and security of supply issues that will face the UK in the future," said the letter.

It warned that climate change is the "most significant environmental threat worldwide", while recent research indicates that, by 2010, the UK will be a net importer of energy.

"Increasing current supply and freezing duty may reduce fuel prices in the short term, but will not address the pressing issue of climate change and the need to reduce our dependence on oil," said the campaigners.

"We believe that the government should make the need to tackle climate change the focus of its response to the current protests."

Protest fears

Meanwhile, the government has slammed the Conservatives for encouraging protests over the price of petrol.

As pressure builds on ministers to prevent further increases in the cost of petrol, transport secretary Alistair Darling accused the Tories of acting irresponsibly.

Darling told the BBC on Thursday that opposition leader Michael Howard was "almost encouraging" disruption on the scale of the 2000 fuel blockades.

"I think Michael Howard in particular is guilty of sheer opportunism, giving a nod and a wink to people, almost encouraging them, when he knows perfectly well that that could cause severe disruption and considerable inconvenience to millions of people in this country," he said.

The Cabinet minister insisted the chancellor would "take a long term view" before making any decision on postponing the planned duty rises this autumn.

On Wednesday Howard said he could see how the rising price of petrol would prompt further protests across the country and his remarks were reported as tacit support for demonstrations.

"I can understand that people might be very angry indeed if the government were to proceed with an increase in fuel duty at this time - it would cause great hardship to many people," Howard added.

"I would entirely understand if they wanted to protest peacefully and within the law."

But the Liberal Democrats branded support for the protestors as "the height of irresponsibility".

"During the last fuel protests millions of people had their lives disrupted and thousands of businesses lost millions," said spokesman Ed Davey.

"For a former home secretary to incite such disruption is astounding. Mr Howard's naked opportunism just days before an election will backfire."