Gail Emms once gave a school presentation about her mum, Janice, who represented England at a 1971 football World Cup in Mexico. The problem was, no one believed her. The idea that a 19-year-old Janice Emms had played in front of 90,000 at the Azteca Stadium in Mexico City, that she stayed in the same hotel as Bobby Moore’s 1970 side, that her team needed police escorts to handle their overwhelming celebrity, was dismissed as fantasy. There was no such thing as a women’s World Cup. Pics or it didn’t happen.

Gail Emms would go on to become a badminton world champion and Olympic silver medallist – and even then she still struggled to convince people of her mother’s achievements. “I’ve been shouting about Mexico for years,” says Emms. “But my mum and her team-mates gave up talking about it because people just thought they were nuts. It wasn’t like you could Google it back then.”

This month, the proof they lacked is finally revealed, in all its joyous colour and raucous crowd noise. Copa 71, a documentary film that tells the story of the pioneering tournament through the players involved in it, is packed with match footage never seen outside Mexico. For Emms, it was a first opportunity to see her mother in an England shirt – “oh my God, there she is, centre forward, hogging the goal!” – and for football, it represents the restoration of one of the greatest lost moments in the sport’s history.

For all the records broken at last year’s Fifa Women’s World Cup, the unofficial tournament staged 52 years earlier remains the best attended in the sport, something Carrie Dunn, – whose book, Unsuitable for Females, describes the history of the women’s game in England – is keen to emphasise.

“After Barcelona’s Champions League semi-final, some pundits tweeted that it was ‘the biggest ever attendance at a women’s football match’, but it really isn’t,” says Dunn. “People look at women’s football and think that nothing happened before the 1991 World Cup final with Brandi Chastain, but we’re still trying to make them recognise that there’s this long, fascinating and important history, full of trailblazing women whose stories have been hidden.”

The 1971 Women’s World Cup was part-miracle, part-powder keg. At the time, the best that female footballers could hope for was to be ignored, as opposed to ridiculed and abused. The FA had only that year lifted the 50-year ban that had barred female players from all its grounds and facilities. Many countries were still copying England’s exclusionary behaviour, and in Brazil it was technically illegal, under martial law, for a woman to kick a ball about.

Yet within this unpromising environment a group of TV executives saw an opportunity. An international tournament held in Italy the previous year had alerted them to the commercial potential of staging an event which – according to their own publicity – combined every man’s two greatest loves: football and women. Across three weeks in August and September, their World Cup would bring together five qualifying teams of England, Argentina, Italy, France and Denmark with hosts Mexico. A well-oiled hype machine flooded the press with stories, selling seats with gimmicks that were as popular – pink goalposts, pop-up beauty parlours – as they were cliched.

One scene in Copa 71, whose co-producers include Serena and Venus Williams, demonstrates just how successful the event marketing was: the England team, most of them teenagers, disembark from their aeroplane to a cataclysm of flashbulbs and wonder who’s on their flight (spoiler alert: it’s them). The documentary highlights the heartbreaking disparity between the way the players were celebrated at the time, both on the pitch and on Mexican TV chatshows, and the vicious reaction to their efforts when they return home. One reason the England team rarely spoke of their achievements was that the FA immediately handed out playing bans for participating in an “unsanctioned” tournament. “Certainly the younger ones had the feeling they’d done something really bad,” says Dunn. “They didn’t even talk about it amongst themselves.”

In 2019, that team were reunited by the BBC on The One Show and dubbed the “Lost Lionesses”. Their story is heartwarming – decades after falling out of touch, they now have a busy WhatsApp group sharing memories and more – but is only one facet of a fascinating tournament that combined dodgy refereeing, pay disputes, player strikes and on-pitch violence. While England deservedly went out in the group stages – losing 4-1 and 4-0 to the significantly more skilled Argentina and Mexico sides – the knockout stages provided one of the most sensational, outrageous World Cup dramas of all time.

Come the second semi-final, all eyes were on Elena Schiavo, a 23-year-old Italian midfielder with a devastating right foot and an even more destructive temper. Mexico had a 2-1 lead when Schiavo scored from a magical free kick some 10 yards to the right of the penalty area – a goal immediately disallowed by the referee. It was, in fact, the second Italian goal the referee had disallowed, and the result was explosive, with furious arguments prompting a mid-pitch brawl.

“If there had been more women football journalists, damn right people would have been retelling that story,” says David Goldblatt, author of The Ball Is Round: A Global History of Football and the sole male voice in Copa 71. “The football world really missed a trick there. At another time, in another place, Schiavo would have a global following.” With her gimlet eye and her “don’t mess with me” attitude, Goldblatt thinks she could have been a proto-Roy Keane.

He believes a renewed understanding and appreciation of moments like these is crucial for the women’s game. “Football is the only popular space I know where people sing ‘Shit club, no history’. Every game played today acquires its meaning, at least in part, through its place in a longer narrative and women’s football has lacked that. Plenty happened between the Dick Kerr Ladies in WW1 and the 1991 World Cup, but no one was telling those stories, and some are epic, mythic.”

“Women’s football has not been chronicled very well until the last couple of years,” agrees Dunn. “It has been difficult to fit these pieces together – you didn’t have official tournaments or professional leagues, or records kept in systematic fashion. There’s often an idea that all the paperwork’s in a box somewhere but no one quite knows where that box is.”

The documentary’s themes of power, control, misogynistic officialdom, and commercial opportunism also resonate strongly in the wake of last year’s Fifa World Cup – and at a time when women’s football is attracting more investment than ever before. “It just proves that football fever has been simmering in the world’s female population for the last 120 years like lava beneath the crust of the earth,” says Goldblatt. “Given any kind of chance, there it was waiting to explode. Isn’t it great finally we’ve got there?”

For the players themselves, the re-emergence of archive footage from the 1971 World Cup has filled in their own personal blanks – Paula Rayner (now Milnes), for instance, had completely forgotten scoring against Argentina in England’s first game. And while their experience was thoroughly fantastical, the documentary allows them to share it with an audience who might never otherwise have believed it possible.

Emms – whose mother inspired her own sporting career – appreciates the chance for the generations that followed in their footsteps to say thank you.

“They did go out to Mexico, and defy everyone, and stick two fingers up to Fifa,” she says. “And it’s really lovely to celebrate that. It wasn’t a dream, it was real.”