In a letter to the EU’s top official on climate action, Frans Timmermans, the dozen EU member states say “the EU can lead by example and contribute to creating the international momentum needed for all parties to scale up their ambition” by adopting a 2030 EU greenhouse gas emissions reduction target “as soon as possible and by June 2020 at the latest”.

This year’s UN talks in Glasgow are crucial, as the world is far adrift of goals set at the landmark 2015 Paris conference, including the aspiration to limit global heating to 1.5C above pre-industrial levels. Even half a degree higher will significantly increase the risks of drought, floods, extreme heat and poverty for millions of people.

The letter piles pressure on Timmermans, who is due to unveil the EU’s long-awaited climate law on Wednesday. A leaked draft of the law shows Timmermans’ plans to propose an EU-wide 2030 target by September. The target would probably be an emissions reduction of 50-55% compared with 1990 levels, which green activists say is not enough to guarantee meeting the EU’s goal of net-zero emissions by 2050.

EU officials think a couple of months’ difference in proposing the target makes little difference, but would allow them to bring onboard more reluctant countries, including Poland, which has not yet signed up to the EU-wide goal of net zero emissions by mid-century.

The climate and environment ministers argue timing is crucial, as they want the EU to have a 2030 target, before an EU-China summit in September and well ahead of Glasgow climate talks in November. “No other major economy is prepared to take the lead to ensure an ambitious implementation of the Paris agreement,” they write.

Wendel Trio, the director of the Climate Action Network Europe, said: “By proposing a 2030 target increase only in September, the commission will give member states no time to reach an agreement by Cop26 in November, the international deadline by which all countries must commit to new, ambitious climate pledges for 2030. The EU needs to have its own house in order, and quickly to push other countries to make substantial contributions well before the deadline.”

The letter, organised by Denmark, was also signed by France, Italy, Spain and the Netherlands. Only two signatories – Slovenia and Latvia – are central and eastern countries that joined the EU after 2004. Germany is conspicuous by its absence.

The EU climate law is the centrepiece of the European Green Deal, which aims to transform Europe’s economy to confront the climate emergency. The law could set Brussels on a collision course with populist governments in Poland, Hungary and the Czech Republic, which have been the slowest moving on the climate emergency. Putting the 2050 net zero target into law means Poland could be outvoted if it continued to refuse to sign up.

Under the law, Brussels would also be able to take a government lagging behind on its climate target to the European court of justice, which can issue daily fines for failure to uphold EU law.

Poland, which generates 80% of its electricity from coal, is seeking EU funds to help wean its economy from fossil fuels. The European commission has proposed a €100m (£87m) “just transition” fund to help countries with coal mining jobs adjust to a green economy, but Warsaw has yet to come onboard.

Climate activists have accused the commission of lacking ambition, as the climate law gives scant detail on how the EU will meet the 2050 net zero target, either by phasing out fossil fuel subsidies, reforming the EU’s common agricultural policy or regulating industry.

“With no 2030 climate target and no measures to end subsidies for fossil fuels, industrial farming and other destructive industries, the commission has left a big hole in what’s meant to be the flagship of the European Green Deal,” said Sebastian Mang, a climate and energy policy adviser at Greenpeace.

The Swedish climate activist Greta Thunberg is likely to give her verdict on the EU climate plans when she meets MEPs on Wednesday. She will also lead a “European strike” in Brussels on Friday with Belgian campaigners.

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