North Sea oil and gas companies that already benefit from huge tax breaks could use fresh rules to slash how much they pay under a new windfall tax announced by Rishi Sunak as part of his £15bn cost of living package, according to a thinktank. The chancellor risks raising a fraction of the £5bn he expects from the complex scheme – which allows the cost of new investments to be offset against profits – should oil and gas companies take the opportunity to dramatically reduce their contribution to the exchequer, said the left of centre Common Wealth. The warning comes after the Liberal Democrats said the chancellor’s refusal until last week to introduce a windfall tax meant he missed out on £3bn from the “extraordinary profits” reported by oil and gas firms in 2021 and another £8bn this year. Christine Jardine, the Liberal Democrat Treasury spokesperson, said Sunak’s “11th hour” 25% windfall tax on oil and gas company profits allowed them to carry on “with business as usual” and direct most of their profits to shareholders. “It’s bad enough that the chancellor waited until the 11th hour to tax big oil and gas, when Liberal Democrats first called for a windfall tax last October. Now it looks like it may not even raise what he said it will. That’s more levy lite than windfall tax.” She accused the chancellor of “going soft on huge companies making a killing out of a crisis”. Oil giants BP and Shell are on course to make a combined profit of about £40bn this year from the rocketing price of petrol and gas. Jardine added: “He chose to leave billions on the table that could have been used for support while slamming families with unfair tax hikes instead. It goes to show how out of touch he and the Conservatives are with people that are suffering.” Common Wealth said North Sea oil and gas firms currently benefited from a range of subsidies to cover the costs of extracting new wells and decommissioning ones that are spent. It pointed to research carried out with the New Economics Foundation, which found that the government had handed firms operating in British waters tax breaks worth about £3.1bn in 2019-20 and £2.5bn in 2020. Most of the funds were directed to shareholders in share buy-back schemes, the thinktanks said. The Treasury has not calculated how much of the £5bn in extra tax could be lost if North Sea operators claim extra investment allowances over the next three years. Labour said the lack of consultation with the industry before Thursday’s statement by Sunak revealed that the plans were hurried and designed to distract from the Partygate scandal. In January, shadow chancellor Rachel Reeves called for a one-year, 10% additional tax on North Sea profits to raise between £2bn and £3bn. Jardine had proposed a 25% tax last October, in line with the chancellor’s scheme, but without the tax breaks on offer from the Treasury.