Climate campaigners are urging the government to set out a clear plan for a low-carbon future in next week’s budget, despite the chancellor’s decision to pull a major plank of climate policy at the last minute.

The budget will determine much of the government’s work this year, and campaigners fear that a failure to send clear signals on meeting the 2050 net zero emissions target would play badly with other countries looking to the UK for leadership as host of the vital UN climate talks, called Cop26, later this year.

Hopes were dealt a blow when it emerged on Friday that a major policy announcement on national infrastructure – including an overhaul of Britain’s energy, transport and water infrastructure to cut greenhouse gas emissions and help cope with the impacts of extreme weather – would be postponed.

“This budget is a big opportunity for the government to set out their stall for Cop26 and lead by example, so it’s disappointing that we may not have a major announcement on low-carbon infrastructure as part of it,” said Doug Parr, the chief scientist at Greenpeace UK.

“But if ministers are using the additional time to improve the plan, that may not be a bad thing. The chancellor should set out a clear timeline for this infrastructure programme and support at least 5% of government spending on climate and nature in the budget. This would show that ministers mean business in tackling the climate emergency.”

Dave Timms, the head of politics at Friends of the Earth, said: “The government reconsidering support for expensive climate-wrecking infrastructure projects such as the £30bn road-building programme would be a welcome sign that international climate commitments are being taken more seriously. However, they need to urgently press ahead with funding a national programme of investment in the UK’s cold and leaky homes where there are clear benefits for the economy, households, and the planet.”

Next Wednesday’s budget will be the first since Britain left the EU and the first since the UK adopted the net zero carbon target, enshrined in law by Theresa May last summer in one of her final acts as prime minister. It will also be scrutinised carefully by other nations for signals that the government is taking Cop26 seriously, after a troubled start to the UK’s presidency.

The EU is setting out its European Green Deal, a massive overarching strategy encompassing all aspects of the European economy, hoped to lead to net zero emissions by mid-century. The EU will use its green deal in a bid to draw China, the world’s biggest emitter, into a pact to cut greenhouse gases drastically.

Without a similar strategy, the UK may lack credibility in trying to form the “grand coalition” of countries that experts say is necessary to make Cop26 – the conference of the parties, to be held in Glasgow this November – a success.

“We’ve been promised a green budget, and with only eight months to go till the UN climate talks, it’s essential we get one,” said Shaun Spiers, the executive director of Green Alliance. “The government is badly off target to deliver its legally binding climate commitments. This is the moment to get on track.”

Campaigners want to see the government commit to a range of measures that would reduce carbon, beyond the piecemeal announcements made so far. Boris Johnson said last month at the launch of Cop26 that the moratorium on sales of new diesel and petroleum cars would be brought forward five years to 2035, and this week ministers agreed to make onshore wind farms eligible for public support, and to increase the content of biofuels in petrol.

Green groups and experts say much more is needed, including strategies on fixing Britain’s heat-leaking homes, boosting renewables, halting the rapid rise in emissions from transport, restoring the natural environment and tackling waste.

“As the first major opportunity for the Johnson government to show it is serious on delivering net zero, the budget is a huge moment ahead of Cop26,” said Jonathan Marshall, the head of analysis at the Energy and Climate Intelligence Unit. “The minimum expected is to lead by example and convince other nations to submit ambitious pledges, amplifying domestic action on to the global stage. The UK can show the world that hitting net zero is eminently possible, and is more than just a slogan.”

Local government is also urging the chancellor to deliver a green budget for Cop26. A group of councils and local government organisations wrote to the chancellor on Friday to say that councils are “uniquely well placed to help the government meet its carbon emissions reduction targets”.

Some groups are calling for the government to devote at least 5% of spending to the low carbon effort.

“Massive investment in our abundant renewable power potential, public transport and a nationwide energy efficiency programme would put the UK at the forefront of the green industrial revolution – and show genuine leadership ahead of this year’s crucial climate summit in Glasgow,” said Mike Childs of Friends of the Earth.

In a sign of the growing impact of the coronavirus on Cop26, the UN announced planned meetings in Bonn in March and April would be cancelled. However the main preparatory Cop26 meeting scheduled for Bonn in June is so far unaffected.

Some also fear that the need for urgent measures on the coronavirus will distract attention from the green agenda.

“The need to avoid recession as a result of coronavirus cannot be made an excuse for dodging vital decisions on de-carbonising the economy,” said Spiers. “Investing in warmer homes, infrastructure for electric vehicles, the renewable energy industry and natural flood defences will boost the economy, not least in poorer regions, as well as tackling the climate and wider environmental emergency.”