The leader of France's main right-wing party on Tuesday backed an alliance with the far right of Marine Le Pen in snap legislative elections, triggering a crisis within his own party and fury from the government.

The stunning announcement by the Republicans (LR) leader Eric Ciotti in a TV interview is the first time in modern French political history that a leader of a traditional party has backed an alliance with the far-right National Rally (RN).

President Emmanuel Macron on Sunday called the elections on June 30, with a second round on July 7, a major gamble after the RN scored more than double the number of votes of his centrist alliance in the EU elections.

With less than three weeks to go before the first round, Macron faces opposition alliances crystallising on the left and right and warnings that his bet could backfire.

A Harris Interactive-Toluna poll published on Monday suggested just 19 percent of people would back him, compared to 34 percent for the far-right National Rally.

But in an interview, Macron ruled out resigning after the election.

The forthcoming ballot has set alarm bells ringing across Europe, as it risks hobbling France -- historically a key player in brokering compromise in Brussels and support for Ukraine against Russian invasion.

"We need to have an alliance while remaining ourselves... an alliance with the RN and its candidates," Ciotti told TF1 television, adding that he had already held discussions with Le Pen, a three-time presidential candidate, and RN party leader Jordan Bardella.

Le Pen praised "the courageous choice" and "sense of responsibility" of Ciotti, saying she hoped a significant number of LR figures would follow him.

Bardella confirmed the alliance, telling France 2 television that his party would be supporting "dozens" of LR candidates for seats.

Eric Zemmour, the former pundit who leads the Reconquest party seen as even more to the right of the RN, applauded the move and called for a "vast alliance of right-wing forces".

- End of 'sanitary cordon' -

The LR traces its history back to postwar leader Charles de Gaulle and is the political home of ex-presidents such as Jacques Chirac and Nicolas Sarkozy.

Now "40 years of a pseudo sanitary cordon -- which caused many elections to be lost -- is disappearing," Le Pen, now head of RN deputies in the lower house National Assembly, told AFP.

But Ciotti's move, which he said was aimed at creating a "significant" group in the new National Assembly after the elections, risks tearing apart his own party.

"I see all those currently agitating for coalitions, for alliances, for little combinations. I'll say right away: I don't believe in it," said Laurent Wauquiez, the leader of the central Auvergne-Rhone-Alpes region seen as a presidential prospect for 2027.

The LR speaker of the upper house Senate, Gerard Larcher, a heavyweight figure, said he would "never swallow" an agreement with the RN and called on Ciotti to resign.

But speaking to reporters after the interview, Ciotti said he would not resign and emphasised that his mandate depended on party activists.

Interior Minister Gerald Darmanin, a past defector from the LR to Macron's alliance, denounced the move as a "dishonour to the Gaullist family" and compared it to the Munich accords with Nazi Germany on the eve of World War II.

- 'Right decision' -

Macron's office delayed until Wednesday a major press conference initially slated for Tuesday afternoon, in an apparent bid to take stock of the realignment of political forces. 

Macron told Figaro Magazine he ruled out resigning, "whatever the result" of snap elections.

"I am only thinking of France. It was the right decision, in the interest of the country," he said, adding that he was prepared to debate head to head with Le Pen.

"I am in it to win," he said.

Prime Minister Gabriel Attal, who media reports said was unhappy with the decision to call the election, broke a two-day public silence to tell TF1 television he would lead the campaign for the ruling party.

Former prime minister Edouard Philippe, who leads a party allied to Macron, told BFMTV earlier that it would "not be completely healthy" for the president to lead the campaign.

France's fractious left-wing parties appeared to quickly set aside differences that had shattered their parliamentary alliance, notably over their conflicting responses to the war in Gaza.

Socialists, Greens, Communists and the hard-left France Unbowed (LFI) said they would "support joint candidates, right from the first round" of the election.