France's prime minister on Thursday warned the French against casting votes for the far right in next month's European elections, saying they risked becoming like Britons who backed Brexit only to have second thoughts when it was too late.

Prime Minister Gabriel Attal faces an uphill struggle to narrow a yawning gap with the far right before France votes on June 9, with polls forecasting a major defeat for the government of President Emmanuel Macron.

Attal accused the far-right National Rally (RN) of pursuing policies that would equate to France no longer being in the European Union, even if the party insists it no longer wants a French "Frexit" withdrawal from the bloc.

"Don't be like the British who cried after Brexit," Attal told RTL.

"When you say you are not going to respect the rules of the single market anymore, not pay France's dues and stop respecting most of the treaties, the reality is that we are no longer in the EU."

Attal referred to recent UK polls showing that, if there was a repeat of the 2016 referendum on EU membership, Britain would vote to stay.

"A large majority of British regret Brexit and sometimes regret voting for something that was negative for their country," he said.

Painting a bleak picture of post-Brexit Britain, Attal said there was "more illegal immigration than ever" and "massive economic problems".

"France will not be stronger by being alone," he said.

The latest poll published Monday by Ifop for Le Figaro showed the RN scoring 33.5 percent in the polls, way ahead of the government alliance based around Macron's Renaissance party on just 16 percent.

Attal, 35, last week went head-to-head in a TV debate with the head of the RN list Jordan Bardella, 28, with government supporters crowing afterwards the premier has exposed a lack of substance in his opponent.

But there has been little sign of the debate making an impression on polls, with questions also asked over why Attal needed to impose himself instead of the little-known head of the pro-Macron list Valerie Hayer by taking on the debate.

Analysts add that the government list is in danger of even coming third in the election behind the Socialist list led by ex-commentator Raphael Glucksmann, which scored 14.5 percent in the Ifop poll.

"Will France be the country that sends the largest contingent of far-right lawmakers to the European Parliament?" asked Attal.

If the far right arrived in force at the parliament, he said, it "could have the capacity to block European institutions, which would lead to very dangerous consequences for our country."

sjw/ah/db