A Venn diagram of those in Labour opposed to Brexit and those critical of Jeremy Corbyn’s leadership reveals much overlap. An outlier is Manuel Cortes. The Gibraltarian is a radical socialist, a decades-long friend of Corbyn and the general secretary of the Transport Salaried Staffs’ Association (TSSA). He also believes that Brexit is a catastrophe that must be stopped.

The trade union leader, who became head of the 22,300-member TSSA in 2011, is a regular platform speaker and op-ed writer but has seldom given detailed interviews. Intrigued by Cortes’s views and his background, I met him for lunch at Haché in Camden Town, north London, a short distance from the TSSA’s headquarters in Euston Tower.

Cortes, whose wiry ponytail and earring reflect his activist heritage, speaks in staccato bursts, never equivocating or wavering in his convictions. “The collapse for the third time of the franchise on the East Coast Main Line just shows that privatisation doesn’t work,” he says of the British railways. “It’s a scam – heads or tails, the private operators [Virgin and Stagecoach] always win… They continue to run other franchises where they’re still making money.” He highlights the irony of Transport Secretary Chris Grayling, a Brexiteer who vowed to “take back control”, allowing “the Dutch state, the French state and the German state” to profit from British franchises. “The only state that is not allowed to run our railways is our own – that is just craziness.”

Cortes was born in 1967 in Catalan Bay, Gibraltar, and raised on the Glacis council estate (“The climate was a hell of a lot better than the UK’s,” he quips of the British overseas territory). His father was an unskilled labourer and his mother a hairdresser.

The family spoke only Spanish at home and Cortes left his English-language school at 15 with no qualifications. But after becoming an apprentice electrician, and achieving a diploma in engineering from the Erith College of Technology in London, he won a place at Edinburgh’s Heriot-Watt University to study electronic engineering. A tutor advised him to improve his English by reading the Financial Times (a title once described by Noam Chomsky as “the only paper that tells the truth”).

As a teenager, Cortes followed the exploits of Nicaragua’s left-wing Sandinistas on Spanish TV and was a founding member and chair of the youth section of the Gibraltar Socialist Labour Party (Labour’s sister party). After securing a Master’s degree in optical electronics from Strathclyde University, he later returned for a second Master’s in economics. Cortes was determined to absorb the theories needed to challenge capitalism. “For many years people have tried to bamboozle me and others in the labour movement, saying that there is no alternative to the way the economy is run.”

Who were his formative influences? “Marx: he understood that capitalism was global in nature and that those people who owned capital really had no nationality. The only way that working people could combat that was by coming together; more practically, people who tried to make it happen, even if it didn’t work. Lenin and Trotsky were very inspirational.”

Cortes, who joined the TSSA in 1998, was one of Corbyn’s most committed supporters in the 2015 Labour leadership election, providing office space and funding (Carmel Nolan, Corbyn’s first head of press, is now the union’s director of communications). “Jeremy’s seen as a beacon of hope, not just in Britain but for working people across Europe,” Cortes said.

It is just before our food arrives (a goat’s cheese burger for Cortes, chicken breast and sweet potato fries for me) that I raise the fraught question of Brexit. Alone among trade union leaders, Cortes has called for the UK to remain in the EU. “Any Brexit deal that introduces friction and borders will finish off the job that Thatcher started because our manufacturing industry will just dwindle away,” he warns. A “soft Brexit” (remaining in the single market and the customs union), meanwhile, would condemn the UK to “vassal statehood” by making it “a rule-taker, rather than a rule-maker”.

Cortes’s antipathy to borders is born of personal experience. The closure of the Gibraltar-Spain border by the Franco regime forced him to make an arduous, day-long trip, via Morocco, to visit his Spanish grandparents (his family could only afford one visit a year). The free movement of people, he argues, is a demonstration of working-class solidarity. When Cortes spoke at a 2013 May Day rally on the Rock of Gibraltar, he declared: “I have more in common with Spanish workers, with British workers, with German workers than with any boss.”

But Labour’s 2017 manifesto pledged to end free movement and Corbyn has refused to endorse a new referendum on Brexit (Cortes was said to be “furious” when the issue was not debated at last year’s party conference). “The Tories are having a conversation with themselves, I think we need to have a conversation with the country,” says Cortes. “Labour is ideally placed to start that conversation.”

Does he believe that Corbyn, a lifelong Eurosceptic, could yet change his mind? “My view is that Jeremy listens to people and he will continue to look at what the facts are,” Cortes says. “And as those facts change, and he continues to listen to people, I’m sure he could change his mind. I see no reason why he would be fixated on any position.”