Mr Jadot, a senior member of France’s Green party, accused Mr Macron of hijacking the European debate to quell the anti-government yellow vest rebellion gripping France.
Asked by Sud Radio whether he agreed with the government’s plan to make the referendum coincide with the crunch EU vote, he said: “There is a risk that the European election will be stolen at the expense of politicising national politics.”
“One has the impression in the end that Emmanuel Macron, like too many European leaders, is abandoning the European battle, is ultimately instrumentalising Europe to solve his own domestic problems.”
He added that a referendum on May 26 would “empty the European election of its contents, kill the national debate on Europe that we so desperately need. I find this incredibly dangerous”.
Mr Jardot continued: “You can see this snare in which Emmanuel Macron is trapping everyone.
“In truth, he doesn’t want the election to be a moment of reckoning for Europe, but a referendum for or against himself.”
France’s conservative weekly Le Journal du Dimanche (JDD) reported on Sunday that Mr Macron was considering organising a referendum in another concession to yellow vest protesters.
Voters are to be asked multiple-choice questions on whether they want to reduce the number of lawmakers, new term limits for elected officials, blank protest votes to be counted in elections and new measures to ensure “citizens’ participation” in public life, top presidential sources told the JDD.
But opposition leaders have since dismissed the popular vote as a red herring to distract from the social crisis triggered by the yellow vest movement, as well as Mr Macron’s refusal to back down on his radical reform agenda.
The yellow vest rebellion – named after the luminous safety jackets all French motorists must carry in their cars – has turned into the most serious political crisis of the Macron presidency.
A 12th consecutive week of rallies on Saturday drew some 58,600 protesters in Paris and other big cities.
The protests began on November 17 as a backlash against rising fuel costs but quickly ballooned into a wider rebellion against Mr Macron’s liberal economic policies, widely perceived as favouring the urban elite over the rural working class.
In December, Mr Macron announced a package of costly measures for low-income families, including a 100-euro (£87.7) hike in the minimum wage, tax cuts for cash-strapped pensioners and tax-free overtime pay.
He then organised a two-month national debate to discuss the government’s policy choices and build a new “contract for the nation”.
The May referendum is seen as a way to conclude the policy debate, and could also serve as a response to the demands from yellow vests for a more participative democracy.
Mr Macron’s inner circle on Monday attempted to play down speculation a citizens’ referendum is on the agenda, with some telling AFP that while “everything is on the table, nothing has been decided yet”.
Prime Minister Edouard Philippe gave a cautious response to the rumours, telling reporters that a referendum was “not yet on the agenda”.
European Affairs Minister Nathalie Loiseau, for her part, said on Sunday that a referendum coinciding with the EU vote could “get in the way,” adding that it would be better for May 26 to be “about Europe”.
Mr Macron told reporters last week that a popular referendum was “one of those things on the table” that “we should reflect on”.
He also supported the idea of holding more referendums, saying that previous French governments had for too long “given the feeling that the elites know better than the people”.
The European elections, which are to be held on May 23-26, are being billed as a bitter clash between liberal europhiles like Mr Macron, and far-right populists calling for a return to national sovereignty and closed borders.