The European Court of Human Rights is forcing 33 governments to prove they are cutting emissions in line with the requirements of the 2015 Paris climate accord.
The court also rejected an attempt by those governments to overturn its decision to fast-track a lawsuit filed by six young Portuguese climate activists.
The activists claim the countries' efforts to cut greenhouse gas emissions are inadequate.
The governments asked the court to drop its priority status for the case and hear their argument that the case was inadmissible, the activists' legal representatives said on Friday.
But the court dismissed the arguments against an urgent hearing and denied their application to defer scrutiny of their climate policies.
The governments now have until May 27 to submit their legal defence.
The activists are aged between 12 and 21.
Four of them live in central Portugal, where bushfires blamed in part on climate change killed more than 100 people in 2017.
The others live in Lisbon, a coastal city threatened by rising sea levels.
Scientists say the man-made emission of greenhouse gases such as carbon dioxide must end by 2050 at the latest to avoid pushing global temperatures beyond the threshold of 1.5°C set out in the Paris agreement.
Gerry Liston, legal officer at the Global Legal Action Network, an international non-profit organisation assisting the activists, said the group would provide evidence that European governments were failing to adopt measures sufficient to meet the requirements of the accord.
The organisation starteda crowdfunding campaign to help support the activists.
They filed their claim last September at the court in Strasbourg, France.
On November 30, the court said it required a prompt response from the 33 countries named in the case, a move that activists said gave heart to their cause.
At the time, the court ordered the European countries to respond to the complaint and granted it priority status because of the “importance and urgency of the issues raised".
The countries named in the complaint are the 27 members of the EU, the UK, Switzerland, Norway, Russia, Turkey and Ukraine.
If the activists win, the countries will be legally bound to cut emissions in line with the requirements of the climate accord.
They will also have to address their role in overseas emissions, including those from multinational companies.