A recent opinion poll from Ifop has given Ms Le Pen a clear lead with 34 per cent of the vote ahead of the first round of voting on June 30, followed by 29 per cent for the hastily-cobbled Leftist alliance, New Popular Front and Mr Macron’s centrists on 22 per cent.
But if the National Rally does finish victorious as predicted, Ms Le Pen will likely hand the prime minister’s seat to her 28-year-old protege, Jordan Bardella.
Mr Bardella told business leaders this week he would cut France’s EU budget payments by up to €3 billion (£2.5 billion) if National Rally wins the snap election next month.
A second poll published last week by Toluna Harris Interactive also predicted the hard-Right party as the clear winner on 34 per cent of the vote.
A third survey by Opinion Way put National Rally on 35 per cent and Mr Macron’s liberal alliance on 20 per cent.
Many of Mr Barnier’s pro-European contemporaries have dismissed the threat of Euroscepticism since Brexit.
Ms Le Pen has sought to tone down its anti-EU “Frexit” rhetoric in recent years as part of a plan to make her movement more palatable to the French electorate.
‘European servitude’
Mr Barnier warned that France should not be complacent about her Eurosceptic intentions, even if she has dropped previous pledges to take the country out of the EU and its euro single currency.
“I don’t think Ms Le Pen and Mr Bardella changed their minds. They are still anti-European,” he said.
“I remember the first sentence of Ms Le Pen on the evening of the referendum in the UK.”
She praised British voters for freeing themselves from “European servitude”, and later described the referendum result as “the most important moment since the fall of the Berlin Wall”.
“I have never heard from Ms Le Pen that she regrets that sentence or that she was wrong,” Mr Barnier added.
“I think she still keeps the same mind… my answer is to say – without any kind of aggression, revenge or satisfaction – look at what has happened in the UK since Brexit.”
But not all hope is lost for Mr Macron, with the current Harris projections not giving National Rally an overall majority in France’s 577-seat National Assembly.
In the French elections, candidates must secure 12.5 per cent of registered voters in order to qualify for a second round of voting on July 7.
While advisers to Mr Macron had feared they risked being squeezed out of a majority of the second-round run-offs, political analysts from OpinionWay said candidates from his party were more likely than socialists to beat hard-Right candidates at this stage.