A former Tory minister who quit the UK Government over a lack of enthusiasm to tackle climate change has joined forces with opponents to urge Rishi Sunak to step up his commitment to the environment.

Zac Goldsmith, who resigned from the UK Government as international environment minister last month over the PM’s apparent apathy towards the climate crisis, has teamed up with cross-party MPs and Peers to demand Mr Sunak amps up action to fight climate change.

Mr Sunak has been sent a letter from the all-party parliamentary group of climate change, signed by 23 UK parliamentarians from all major political parties including SNP MPs Tommy Sheppard and Martyn Day and former Labour environment secretary Hilary Benn.

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The letter calls on the Prime Minister to hold firm on the UK Government's net zero commitments after suggestions he may retreat from green policies.

It comes after some Conservative backbench MPs called on Mr Sunak to think the pace of his net zero strategy.

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The letter also calls on the Prime Minister to commit to attending the upcoming UN climate summit COP28 and reject the expansion of new fossil fuel infrastructure including the controversial Rosebank oil field, which they say will do nothing to enhance energy security or save households money on bills given the majority of oil from the field will likely be exported.

Pressure is mounting on the UK Government to reject the Rosebank plans amid speculation a decision on granting permission has been delayed over climate targets.

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The UK Government's net zero tsar, Chris Skidmore, asserted the development is not compatible with net zero, while former Climate Change Committee chairman and ex-Tory environment secretary, Lord Deben, argued that the expansion of new oil and gas in the UK is “unnecessary” and “hypocritical”.

The letter states that COP28 “must be the moment that the global community agrees to urgently phase out fossil fuels”.

It adds: “For this to happen, the UK has an important role to play in leveraging its international influence, and working constructively with all parties, to help secure an agreed package that clearly names the requirement to phase out all fossil fuels and set goals for the upscaling of renewables.”

The group is calling on Mr Sunak to “support our allies in calling for an end to the fossil fuel era and move more quickly towards a clean energy world” and join “other countries such as Denmark, France, Germany, and others in championing the need for an ‘urgent phase out of fossil fuels’.”

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The letter adds: “As recommended by the CCC, the UK should also set out a clear position and plan to move beyond oil and gas through a just transition, and strengthen the UK’s language on this in all international climate fora.”

The Prime Minister has also been urged to “demonstrate leadership by taking action at home including by rejecting the expansion of new fossil fuels which the CCC is clear are not compatible with net zero”.

The letter says: “In line with the report’s arguments, Lord Deben (the CCC’s outgoing chair), has asserted that new fossil fuel developments are both ‘unnecessary’ for the UK and a ‘bad example’ to the world.

“In light of this, we believe that the Government must reject new fossil fuel infrastructure including the Rosebank oil field which will do nothing to enhance energy security because the field is 90% oil, likely for export, and therefore won’t save households money on their energy bills either.

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“Instead, the Government should concentrate its efforts on making action on net zero easier, including by lifting the ban on onshore wind, embedding a net zero test across government and within the planning system, and accelerating the roll-out of energy efficiency measures which will bring bills down permanently.”

The UK Government has committed to become net zero by 2050, five years after Scotland’s 2045 pledge.

Mr Sunak, who was previously criticised by Lord Goldsmith for being “uninterested” in climate change, said he did not want to “hassle” families with extra costs and would pursue net zero policies that were “proportionate and pragmatic”.

Comments from Downing Street suggested that, while the UK Government was still committed to policies such as phasing out gas boilers and ending the sale of petrol-powered cars, it would keep those policies under review in light of costs and changing technology.