The UK government could face a fresh legal challenge over fears that ministers may wave through plans for oil drillers to keep exploring for new North Sea reserves without considering the latest evidence of its impact on the environment. Lawyers from ClientEarth, an environmental law charity, wrote to Kwasi Kwarteng, the business secretary, days before the start of the Cop26 climate summit in Glasgow to warn against approving projects such as the controversial Cambo oilfield development by relying on outdated climate checks. In the letter dated 29 October, seen by the Guardian, ClientEarth warned the government that any decision on offshore oil and gas developments must consider their full climate impact or its lawyers would be “prepared to challenge” ministers through a judicial review. Sam Hunter Jones, a lawyer at ClientEarth, accused the government of being “asleep at the wheel” and said ministers risked making decisions based on out-of-date climate assessments unless it adopted new criteria reflecting growing international evidence against new fossil fuel developments. The government was due to set out new environmental assessment criteria to guide decisions on whether to allow North Sea projects before the end of the year, but has not issued an update since opening a consultation in May. “Ahead of Cop26, where the UK government is positioning itself as a leader on climate action, it’s difficult to understand why work on this vital assessment is not moving forward with urgency,” Hunter Jones said. He said the impact assessment currently being used was “severely out of step with the recent authoritative warnings on the action necessary to avoid catastrophic global heating”. Ministers announced in the spring that they would allow oil drillers to keep exploring the North Sea for new reserves, despite the government’s pledge to reduce UK carbon emissions to net zero by 2050, as long as they passed a “climate compatibility” test in addition to the existing environmental checks. It later emerged that a major new development backed by Royal Dutch Shell at the Cambo oilfield in the North Sea would not be required to face the compatibility test because technically the plans were an “extension” to an existing licence. Hunter Jones said decisions on new North Sea oil and gas projects like the Cambo oilfield “risk fatally undermining efforts to meet the world’s climate targets by locking in polluting infrastructure and emissions for decades to come”. The government already faces a potential legal threat from Greenpeace over its refusal to rule out the Cambo oilfield, which triggered an outcry from climate experts and green campaigners in Scotland. The Cambo field is expected to produce 170m barrels of oil in its first phase, or the equivalent of the annual emissions of 18 new coal-fired power plants, according to Friends of the Earth. A stark report from the International Energy Agency this year said there could be no new development of oil and gas fields if the world was to stay within safe limits of global heating and meet the goal of net zero emissions by 2050. A government spokesperson said the UK could not have “a cliff-edge where oil and gas are abandoned overnight” and that if the UK stopped producing gas “this would put energy security, British jobs and industries at risk, and we would be even more dependent on foreign imports.” Official figures show that the UK’s reserves stood at 4.4bn barrels of oil at the end of last year, or enough to sustain production until 2030 without any additional exploration. “While we ramp up renewable energy capacity, this ongoing but diminishing need for oil and gas over the coming years will continue,” the government spokesperson said.