Pressure mounted on Gordon Brown last night to slap a multi-billion-pound windfall tax on the UK's oil companies to provide new money for vote-winning populist measures such as tax cuts.

Yesterday, the Treasury maintained the line of not ruling out such a windfall tax in the autumn's Pre-Budget Report (PBR). Introducing one would horrify the oil barons but delight the Prime Minister's Labour colleagues.

For weeks, back benchers in the governing party have been calling for Mr Brown to sting the oil companies which, they argue, are set to make large profits once again on the back of rising oil prices.

In January, Royal Dutch Shell reported annual profits of £13.9bn, a record for a UK-listed company. Much of the rise was put down to rising oil prices, which at the time stood at $91 a barrel compared to around $130 now.

Derek Simpson, joint general secretary of Unite, which, with 1.9 million members, is the largest financial contributor to Labour, insisted the PM had to "reconnect" with the concerns of the party's traditional core vote if its fortunes were to revive following the crushing by-election defeat at Crewe and Nantwich.

"How popular do you think it would be, given that oil companies are raking in billions, if he imposed a windfall tax on them and distributed it through something like a council tax cut?" asked Mr Simpson.

In a speech to his union's conference in Brighton today, the union chief will insist a windfall tax on oil companies' profits would "redefine" Mr Brown's premiership and win back votes for Labour.

In 1997 after Labour came to power, the then government levied a windfall tax on the energy companies, raking in £5.2bn for policies to boost employment.