Pressure is mounting on Gordon Brown to delay plans for a further two pence-a-litre increase in petrol and diesel duty in September.

Fuel campaigners announced plans for a peaceful protest by hauliers in south Wales on Saturday and a slow-moving convoy of lorries in Newcastle upon Tyne next Wednesday - the eve of European, mayoral and local elections.

Meanwhile, sparking claims of opportunism, Michael Howard signalled that the Conservatives are ready to support fuel protests provided they were within the law.

The Tory leader said: "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."

Asked whether he would back protests, he said: "As long as they are peaceful and within the law, they may well be supported."

Labour and the Liberal Democrats accused him of "irresponsibility" in supporting action which could disrupt people's lives.

Alistair Darling, the transport secretary, accused Howard of "complete opportunism".

"He knows what is driving up prices at the pump is the high international oil price," Darling said.

The Express and Mail report that Britain could be put on a three day week under secret government plans to tackle a new fuel crisis.