EU negotiators have been struggling for weeks to finalise a harmonised classification system for green finance in Europe that could decide the fate of hundreds of billions of euros in investment.

European Central Bank chief Christine Lagarde has underlined the importance of the reform, with sustainable finance deals reaching one half a trillion dollars in 2018.

But the long-standing disagreement over nuclear energy has undermined the EU's efforts to cut greenhouse emissions, with a promise last week by EU leaders for carbon neutrality by 2050 nearly scuppered by a feud over atomic energy.

The lobbying frenzy in Brussels over the new EU norm has been immense, with soon to Brexit Britain also making its voice heard while protecting the interests of the City of London financial hub.

"This is a historic moment... the much-needed enabler to get green investments to flow and help Europe reach climate neutrality by 2050," said EU Commission Vice President Valdis Dombrovskis.

Late on Monday, EU lawmakers approved an offer by member states that delayed the nuclear question -- as well as the role of natural gas cherished by Berlin -- until the end of 2021.

"I am fully aware that the nuclear problem will return in two years' time. We pushed back the matter," said the chairman of the European Parliament's Environment Committee, French centrist MEP Pascal Canfin.

"The risk was to take the whole classification hostage," he added.

Ever since the European Commission's proposal was put on the table in May 2018, nuclear energy has been the subject of a huge fight between its supporters, led by France and backed by Eastern European countries.

But opponents of nuclear power -- such as Germany, Austria, Luxembourg and Greece -- have refused to back down, with domestic opinion fearing atomic energy disasters, such as Fukushima or Chernobyl.

The compromise suggested by Finland, which holds the EU's rotating presidency, was reached with MEPs behind closed doors and needs final approval by member states envoys on Wednesday.

Once approved, the European Commission will then have two years to draw up detailed lists of sectors eligible for a Green finance label, based on the criteria.