French President Emmanuel Macron's promise to enshrine the fight against climate change in the constitution via a referendum was in doubt today as senators appeared poised to torpedo the plan.

The initiative to state in the constitution that France "guarantees environmental protection and biological diversity, and combats climate change" originated in a citizen's body set up by Mr Macron last year.

Seeking the upper hand in what could be a key issue in next year's election, the president promised a referendum on the bill if it gained approval in both houses of parliament.

The National Assembly, where Mr Macron has a majority, overwhelmingly voted in favour of the revision in March.

But on Monday, the bill goes to the Senate, where the right-wing Republicans hold sway.

They have already decided "to empty the bill of its substance", Pieyre-Alexandre Anglade, a National Assembly deputy for Macron's LREM party, charged on Sunday.

He warned in the JDD weekly that changes "will prevent the agreement," as under French law a referendum can go ahead only if it is approved in identical wording by both houses of parliament.

"As things stand, there can't be a referendum," National Assembly speaker Richard Ferrand acknowledged Sunday. A majority in the Senate, he said, was "more conservative" concerning environmental questions.

But Mr Macron's office told AFP that the plan to change to constitution was "in no way buried".

Government spokesman Gabriel Attal added that "obviously we want the referendum to happen, this is what the president committed to".

Mr Attal said the "ball is in the Senate's court" and "that there seems to be a wish by senators to torpedo" the initiative.

A majority of senators takes issue with the word "guarantee" in the bill, which they say implies that environmental concerns would take priority over other constitutional principles.

JDD meanwhile, in a separate article citing anonymous sources, claimed that Macron had already given up on the referendum idea even before the Senate vote.

Condemnation by his political opponents was swift, with Green party boss Julien Bayou saying that Mr Macron "as usual made a promise he couldn't keep".

Centre-right senator Bruno Retailleau said Macron was guilty of "hypocrisy", saying the president "accuses us of obstruction to justify the cancellation of a referendum that he never wanted".

Green deputy Matthieu Orphelin said the government had refused to negotiate with the Senate, thus "manoeuvring so the process wouldn't succeed".

Meanwhile thousands of people began to demonstrate in Paris, demanding further-reaching legislation against climate change, as part of protests and meetings planned in 150 French cities today.

"We denounce the lack of ambition in the climate law and, since this morning, the near-certain cancellation of the referendum, which constitutes another step backwards," Cyril Dion, a member of the climate citizen's group, told AFP at the protest.

The last referendum in France was in 2005, when voters were asked to back the creation of a European constitution.

It was rejected in a humiliating defeat for then-president Jacques Chirac.