In France, as in so much of Europe, the traditional centre left has collapsed. The once formidable Socialist Party appears in irretrievable decline after the worst election result in its history in 2017 (its presidential candidate Benoît Hamon finished fifth with just six per cent of the vote).

One of the chief beneficiaries is Jean-Luc Mélenchon, the leader of the left-wing France Insoumise (France Unbowed) and the most notable international speaker at this year’s Labour Party conference. When I met the 67-year-old (dressed in his trademark Maoist jacket) in Liverpool he glanced approvingly at a new poll showing that he is now France’s most popular politician. Support for president Emmanuel Macron, the liberal centrist elected last year, is now below that of his Socialist predecessor François Hollande at the same point (just 28 per cent approve of him).

I asked Mélenchon - who finished fourth in the presidential election with 20 per cent - why Macron had become so rapidly unpopular. “His social policy, of course, but also his arrogance and his way of talking to ordinary French people, no other French president - or even king - had ever spoken to French people like that,” the National Assembly member said. The president recently told an unemployed gardener that he could find a job if he merely “crossed the street”.

Mélenchon added: “Macron treats people as ‘do-nothings’, as Gauls resistant to change. He said that a train station is an incredible place because you meet people who succeed in life but also people who ‘are nothing.’

“In very little time, he’s come to appear as a man of the right ... There was never a social base for Mr Macron’s project, he only just sneaked ahead in the vote - 24 per cent to my 20 per cent. They succeeded in a hold-up [Macron faced the far-right Marine Le Pen in the run-off], as they themselves say.”

Mélenchon, who had never previously attended Labour conference, told me that Jeremy Corbyn was “unique in Europe” because “it’s only the case in which an alternative has arisen within a socialist party and won.”

By means of an internal revolution, Corbyn has saved Labour from the decline - or Pasokification (a reference to Greece’s vanquished Pasok) - of its European sister parties. In France, by contrast, Mélenchon said, “it was necessary to go around the Socialist Party and, indeed, defeat it to make the alternative possible”.

The “red thread” that united him and Corbyn, he added, was “people’s will to take back control of their situation when faced with bankrupt parties, institutions and figures ... this is an extraordinarily powerful political force and in France it’s exploded the traditional political terrain.” He observed: “If the Brits want to nationalise the trains, it’s because the trains don’t work, it’s not an ideological motivation.”

Jean-Luc Antoine Pierre Mélenchon was born in Tangier, Morocco, in 1951 to parents of Spanish and Sicilian descent (his father was a postmaster and his mother a primary school teacher). The family emigrated to France in 1962 and Mélenchon studied philosophy at the University of Franche-Comté.

After joining the Socialist Party in 1976, he later served as a senator for nearly 20 years and as minister for vocational education from 2000-02 in Lionel Jospin’s administration. But he resigned in 2008 in protest at the party’s rightwards drift and founded the rival Left Party, before launching France Unbowed in 2016.

Mélenchon’s policies include a 100 per cent tax on incomes above €400,000, the introduction of a four-day week, French withdrawal from Nato and the establishment of a Sixth Republic (to end France’s monarchical presidency). He also supports the country’s ban on wearing the burqa in public, a stance which led some to condemn his invitation to Labour’s conference.

“In France it’s illegal to walk naked in the street, nobody ever asks me what I think about that, no one finds that offensive,” Mélenchon remarked when I raised the subject. “To walk in the street entirely covered is a denial of the human right to see someone’s face.”

Though Mélenchon is a disruptive political presence, he takes a traditionalist stance on French secularism. “We’re secularists and we’re attached to the secularism of the French state: no politics in religion, no religion in politics. I don’t have a position on Islam just as I don’t have a position on Catholicism, Judaism and Protestantism. You’ll never know if I have a religion and, if so, what it is, because I’m elected for all French people.”

He added: “In France we had three centuries of open or latent war between Catholics and Protestants and, before that, eight centuries of persecution against Jews. Religion is the worst disaster that can hit politics. For us, it’s a personal, political and philosophical decision to say that religion is a private affair that has nothing to do with politics.”

Nevertheless, to most at the conference, Mélenchon’s insistence that religion is a “private” matter is contradicted by his support for a state-mandated dress code.

Like Corbyn, Mélenchon also stands in a tradition of left Euroscepticism: he has vowed to renegotiate the EU’s “neoliberal” treaties if elected and supported a No vote in the 2005 French referendum on the proposed EU constitution.

He said of Brexit: “The people in Britain have voted for it, so what’s the problem? I warn you that those who deny a democratic vote that’s taken place will create interminable problems in their own country, that’s what happened in France and Holland. Because we voted ‘No’ to the EU constitution and the elites thought they’d heard ‘Yes’, so they applied the treaty all the same. Since 2005, France has never forgiven this theft of democracy.”

Mélenchon is hopeful that Corbyn will become the first socialist leader to take office in a major European country since François Mitterrand in 1981. “He’s certainly capable and for us that would be extremely gratifying.”

What of his own presidential chances? Could he occupy the Élysée Palace by 2022? “You can never be confident in an election, you have to look for the result ‘with your teeth’, you have to struggle for it,” Mélenchon replied. “We’re a very lively and rebellious people and therefore an unpredictable one - almost as much as the Brits.”

But of one thing he is sure. Before the 2012 presidential election, the blandly social democratic François Hollande reassured the City of London: “I am not dangerous”. By contrast, Mélenchon told me as he departed: “I am very dangerous.”