Boris Johnson’s much-vaunted green recovery plans are inadequate and “clearly unlawful” as they do not match up to the government’s legal obligations under the Paris climate agreement and the UK’s own net zero emissions target, green campaigners have said. On Tuesday, a letter threatening court action was sent to the prime minister and the chancellor, Rishi Sunak, by the pressure group Plan B, which successfully took the government to the appeal court this year over its green light for the expansion of Heathrow airport. Sunak will set out £3bn of green spending, focusing on improving energy efficiency in homes and public buildings, in his summer statement on Wednesday. But the letter contrasts this sum with the billions committed to airlines and carmakers in the taxpayer-funded coronavirus recovery package, and funding for fossil fuels. “The proposed approach is quite clearly unlawful … it is no more than a fig-leaf for the government’s new deal for polluters,” wrote Plan B in a letter before action, a legal first step that gives ministers a chance to reply before instigating formal legal proceedings. If there is no response by 17 July, the campaigners will take the next step, which is to send a “pre-action protocol letter”, which would oblige the government to respond within 21 days. The campaigners argue that the Heathrow case – in which the government’s go-ahead for a third runway was deemed unlawful by judges as it failed to take into account the UK’s obligations under the 2015 Paris agreement – sets a precedent that forces ministers to assess the impact of their Covid-19 stimulus plans on the climate crisis. Legal action is also threatened against the Bank of England, alleging that the governor, Andrew Bailey, appeared to have changed his mind on how to combine responses to the Covid-19 crisis and the climate emergency. In a statement on 1 July he said: “The Bank’s lending to companies as part of the emergency response to Covid-19 has not incorporated a test based on climate considerations. That was deliberate, because in such a grave emergency affecting this country we have focused on the immediate priority of supporting jobs and livelihoods.” Plan B said this showed he had been overruled by ministers. Tim Crosland, its director, said: “It feels like his judgment has been overridden. Corporate lobbyists are getting the better of the scientific and economic advice. This is a disaster.” A spokesperson for the Bank pointed out that the governor’s statement also confirmed the Bank’s commitment to countering climate change. A government spokesperson said: “Throughout this [coronavirus] crisis, we’ve continued to take our environmental responsibilities seriously and remain committed to meeting our climate change and wider environmental targets, including net zero [greenhouse gas emissions] by 2050. Tackling climate change is at the heart of our economic recovery and the announcements expected in the summer economic update will follow on from those in the prime minister’s new deal speech last week, including reforesting Britain and making additional funding available this year to attract investment in green technologies.” The threat of legal action came as the Treasury set out further details of the planned £3bn of green spending. Under a new home insulation scheme, from September households will be able to apply for vouchers covering the cost of measures such as cavity wall or loft and floor insulation. About £1bn will go in grants of up to £10,000 for the poorest households, while a further £1bn will be available in vouchers of up to £5,000 covering two-thirds of the cost for other homeowners. It is unclear what support may be available to private renters, who make up a large proportion of the UK’s households but are usually forbidden from undertaking improvements. Households could save as much as £600 a year on their energy bills, according to the government, and the scheme, known as the Green Homes Grant, could support 100,000 jobs. Campaigners gave a cautious welcome but stressed that key details were still unclear, and that the initial support of £3bn for this year, which includes £1bn for decarbonising public buildings such as schools and hospitals, would need to be carried on in future years to refit all of the UK’s draughty housing stock. Previous unsuccessful schemes have been stop-start, subject to tinkering or failed to offer enough incentives to cash-strapped households. Mike Childs, head of policy at Friends of the Earth, said: “To make a difference, this money needs to be around for the long term so the industry can mature. It would need to be closer to £5bn to achieve significant improvements to the UK’s leaky and inefficient housing, and fit them with eco-heating.” Caterina Brandmayr, of the Green Alliance thinktank, said: “It’s particularly encouraging that a large share of the funding will support lower income households, providing warmer homes and cheaper bills. Effective delivery will also rely on government communicating the benefits of home insulation and supporting jobs through a new training and skills programme.” Previous schemes also struggled because homeowners were reluctant to undergo the inconvenience of building work, and because the financial outlay required was too great. Caroline Lucas, the Green party MP, warned: “When money is tight for so many and unemployment is rising, I’m concerned that even when people are prepared to clear out their lofts to install insulation, or get their windows replaced with double or triple glazing, many won’t risk spending their savings to do this and our housing will continue to pour carbon emissions into the atmosphere.”