Keir Starmer has described nuclear power as a “critical part” of the UK’s energy mix and pledged to get stalled projects over the line, as Labour positions itself firmly behind the technology. Before the Labour leader presents his net zero energy policies during a speech in Scotland early next week, the party said it would “push forward” nuclear to boost energy security, cut costs for consumers and create jobs. On Sunday Starmer accused the Conservatives of a “shambolic” failure to open any nuclear power plants during 13 years in power. On Monday he will visit Hinkley Point C in Somerset, one of a number of new plants still not operational despite having been identified in 2009 when Labour was last in power. “My government will lower household energy bills, create jobs and ensure Britain’s energy security. Nuclear is a critical part of the UK’s energy mix,” said Starmer, who claimed Tory failure to approve the new plans had cost 7,000 British jobs. However, another plank of the party’s energy plans came under renewed attack on Sunday from a union that is one of Labour’s biggest donors. Gary Smith, the GMB general secretary, accused the Labour party of being naive over its plans to ban North Sea oil and gas production. Labour has pledged to block all new domestic oil and gas developments if it wins power, proposing instead to invest heavily in renewable sources such as wind and also in nuclear power. The proposals, which Starmer is expected to set out formally on a visit to Scotland this month, will involve not only a ban on new North Sea oil and gas licences but also a pledge that any borrowing for investment should be limited to green schemes. Smith said Labour had “got it wrong” and risked creating “a cliff-edge with oil and gas extraction from the North Sea”. “We are critical friends of the Labour party and I think this is just a lack of intellectual rigour and thinking about where they have got to on oil and gas,” he told Sky’s Sophy Ridge on Sunday. “They are focusing on what they think is popular rather than doing the proper thinking to understand what is right for the country.” The shadow business secretary, Jonathan Reynolds, said earlier on the same programme that the existing oil and gas fields in the North Sea would run until 2050 and the party was not talking about turning those off. Starmer’s promise to block new North Sea oil and gas exploration has received the backing of an eclectic range of high-profile groups, including environmental campaigners, trade unions and even the Women’s Institute. The radical pledge is a key plank of Labour’s environmental platform, but has angered business and political leaders in the north-east of Scotland, where the industry is concentrated. Starmer is being urged to stick to the plan in a letter signed by 139 organisations, including the Countryside Charity, the Royal Society for the Protection of Birds and the National Federation of Women’s Institutes. The groups write: “We urge you to stand firm on Labour’s policy of no new oil and gas developments and its significant investment in well-planned, nature-positive renewables and energy efficiency, and to confirm more details on how Labour will support workers to transition from fossil fuels to good quality, secure green jobs during your speech in June.” Shadow ministers confirmed last month that the party intended to ban new domestic oil and gas developments as part of its strategy to achieve zero-carbon power by 2030. Starmer will formally announce the pledge as part of a speech later this month setting out his green agenda in more detail. The plans have drawn fire from Aberdeen-based business leaders and the Tories, with Grant Shapps claiming the policy was an “ideological vendetta against British energy independence” and warning it would put jobs at risk and boost Russia’s global power. Shapps, the energy secretary, is due to make a decision within weeks over whether to approve drilling at the giant Rosebank oilfield off Shetland. Comments from Rishi Sunak – who has said it would be “economically illiterate” not to invest in new UK oil and gas – have been interpreted as a sign that the government is prepared to give it the green light, although there have been warnings that Labour’s green policies could make it less attractive to investors. In an article for the Times, Ryan Crighton, the policy director of Aberdeen and Grampian Chamber of Commerce, said: “If the alternative is importing, at a greater carbon cost, then surely the UK should always favour domestic production, where we can control the regulatory environment.” The high-profile opposition to Labour’s plans has caused concern among environmental groups, with Labour recently having backtracked on a range of other policies, including a pledge to abolish university tuition fees. Starmer is also under pressure from some of his frontbench to change the remit of the party’s £28bn climate fund to allow it to invest in infrastructure projects that are not explicitly green in nature. Tessa Khan, the founder of Uplift, a group that campaigns for the UK to move away from fossil fuels, and which signed the letter, said: “We were disturbed by the way in which Labour’s position to oppose development of new oil and gas field has come under attack in the last week. We wanted to make it clear that it is a core position of the climate sector and more broadly among different sectors in the UK. It’s a mainstream position and backed up by climate science.”