It's 14 years now since a young film-industry executive working as head of development for David Heyman's London-based production company Heyday Films spotted a newspaper article about a first-time novel describing the adventures of a little boy at wizard school. The 29-year-old executive's job was to source British material that Heyday Films could option and then sell on to Warner Bros. She was looking for something that might have "longevity". This was in 1997, before JK Rowling's Harry Potter books had become international bestsellers. When she saw the article saying that the woman was planning a series of seven novels, she was immediately curious.

"If I hadn't read that article on that day, there is no way that David or I would have been involved in it," the executive, Tanya Seghatchian, later stated. "We were very lucky – we got there much earlier than anyone else did."

Speaking this week, Seghatchian (now head of the Film Fund at the BFI) recalls that she thought the idea of "a little boy who goes to boarding school was a uniquely British situation". The notion of wizardry and magic, she hoped, might appeal to a big studio like Warner Bros. Equally pertinently, she was moved by the novel in a way that she had not anticipated. "When I got to the chapter where Harry looks in the mirror and sees his mother – and he's obviously an orphan child who had grown up without her, in this ghastly cupboard under the stairs, with an aunt and uncle who treated him in the way that they did – I was profoundly moved. I started welling up. I had tears in my eyes. This emotional reaction suggested the universality which is bigger than just a children's film."

At the time, in the late 1990s, family film wasn't as all-pervasive as it is today. Disney had few rivals. However, Potter was to give Warner Bros the chance to enter this area.

Seghatchian had set in motion what has become one of the biggest juggernauts in film history. Now, with the release of the final Harry Potter feature, Harry Potter and the Deathly Hallows: Part 2, in mid-July, with the London premiere next week, that juggernaut will finally screech to a halt. After eight movies in 10 years, the Potter saga has run its course. Potter faces his final battle with Lord Voldemort and his last chance to save Hogwarts School of Witchcraft and Wizardry. "IT ALL ENDS 7.15", says the slogan on the poster, which shows Harry in a brown leather jacket against a rubble-strewn backdrop, looking like a teenage Mad Max in specs as he prepares for his last, and defining, tussle against the Dark Forces.

In Britain alone, around 10 million people have seen each new Potter film. The overall UK admissions once the new film is released should stand at approximately 80 million. Cinema exhibitors around the world are currently fretting about what will plug the $7bn hole that Potter's disappearance will leave in their release schedules. The flip answer that something else will simply turn up ("it always does") in Potter's place doesn't wash.

"It has been a really remarkable run. To have achieved that number of films in that period of time on the scale of those undertaken has been without precedent anywhere in the world," notes Mark Batey, Chief Executive of the UK's Film Distributors' Association Ltd. "It is a phenomenon. It has really crossed over from cinema screens into every walk of life – every bus conversation, every family breakfast table. Harry Potter is part of our lives now. It is a real cultural force."

Creating successful film-franchises from children's books is an extremely delicate and risky business. The cautionary tales of franchises that have stalled after one outing underline the challenges that producers face. For every Harry Potter or Twilight, there is a Stormbreaker or Golden Compass.

One of Potter's trump cards has been its idealistic and painstaking approach toward its source material. The Mexican director Alfonso Cuaron picked up on this when he was hired to make Harry Potter and the Prisoner of Azkaban (2004). He hadn't read the Potter books or seen the two earlier films. "I was sceptical and amused," he admitted. "The stupid, cynical part of me thought it was just a machine to squeeze money out of families and children – like a cheap franchise."

Cuaron was soon won over: he realised that the team behind Harry Potter was determined to capture "a sense of wonder" on screen, not simply to adapt the Rowling books in a rigid and formulaic way. It was apparent that screenwriter Steve Kloves (previously best known for scripts like The Fabulous Baker Boys and Wonder Boys) had worked very closely with Rowling.

The casting was pivotal too. The producers continually hired the very best British character actors. These performers may often have lurked behind layers of make-up and lanky or thick wigs but they were always recognisable – and they were always given at least a few lines of dialogue. Whether it was Alan Rickman as the purring and thoroughly malevolent Severus Snape or Ralph Fiennes as the sneering, misshapen Lord Voldemort, Helena Bonham Carter's Gothic and scatty Bellatrix Lestrange or Imelda Staunton as the demurely evil Professor Dolores Umbridge, the baddies seemed like a cross between Shakespearean villains or characters on leave from a Charles Dickens novel. There were also plenty of comical "rude mechanical" types: Brendan Gleeson in roistering form as Alastor "Mad-Eye" Moody, Robbie Coltrane as the impossibly hirsute Hagrid, etc.

The kids were personable and very British. Daniel Radcliffe's earnest, bespectacled Harry Potter, Rupert Grint's ginger-haired Ron Weasley and Emma Watson's pert and pretty Hermione were surely easier for younger viewers to identify with than the blandly good looking leads in so many American kids' and teen movies. It was fascinating, too, to see them grow up.

Another of the series's strengths was its willingness to grow more brooding and complex as the series progressed and the main characters entered adolescence. This has been reflected in the films' classifications. Whereas Harry Potter and the Philosopher's Stone was a PG, the more recent instalments have all been 12A.

According to the BBFC's "extended classification report", The Deathly Hallows: Part 2 is "quite dark and there is a sense of threat throughout." Several leading characters are killed and the BBFC highlights one sequence "when Voldemort orders the murder of a key character, leading to them being repeatedly bitten by a snake. We hear the attack rather than seeing it, but there is quite a lot of blood on the victim's neck and on the wall behind him afterwards". There is another "gory moment" when Harry enters a dream-like world "and sees a bloody foetus-like creature".

In its discussion of The Deathly Hallows: Part 2, the BBFC highlights what is both one of the franchise's strengths and (to non-Potter devotees) one of its greatest frustrations. Yes, the BBFC acknowledges, there are "sad moments" and smatterings of violence throughout but suggests "that many viewers will already be familiar with the story from the books, reducing the impact". As this remark underlines, Potter films are primarily for the converts. Anyone watching The Deathly Hallows: Part 1 who didn't have at least a smattering of basic Hogwarts knowledge would have been deeply baffled by so many characters whose back stories weren't explained at all. That this wasn't an issue suggests that almost everyone who went to see the movie was at least partially steeped in Potter lore. It also underlines the confidence of the producers.

The franchise has also benefited from its religious neutrality. The Potter films were never ensnared in controversies like the one that dogged The Golden Compass (2007.) Made by New Line, the company behind The Lord of the Rings, this had been intended as the first in a series of movies based on Philip Pullman's books. However, after the Catholic League called for a boycott of the film, the sequels were scrapped. Although the producers had gone to some lengths to water down the anti-Church elements in the novels, they still fell foul of the religious right in the US. "Hollywood opens the Antichrist door" and "atheist propaganda for children" were among the charges made against the film. At the same time, the producers risked antagonising the fans of the novels by making too many changes. The film did relatively well internationally, but its hopes of Harry Potter-like longevity were scuppered by its disappointing US performance.

The Harry Potter series has been able to draw on Warner Bros' huge marketing muscle – way beyond anything independent distributors could afford. In the early years of the movie franchise, the books were still being published. Warner Bros and the books' publisher, Bloomsbury, were able to cross-promote their wares. Their scheduling of the film releases was exemplary too. Potter movies always appeared either in mid-July or mid-November, just before the summer or Christmas holidays.

This combination of timing, luck and marketing ingenuity was something that Potter's rivals couldn't match. Stormbreaker (2006), the adaptation of Anthony Horowitz's novel about teen spy Alex Rider, had briefly seemed a likely pretender to Potter's crown. Horowitz's books had a fervent following. The Weinstein Company had bought the US rights. The film itself was entertaining enough. However, it quickly became clear that there wasn't anything like the marketing power behind Stormbreaker that had been propelling Potter. The film underperformed so badly that it didn't even make back its production budget and plans for sequels were quietly shelved.

The question is whether the Potter films will be discovered, and rediscovered, by new generations without new instalments of the Potter saga to lure them to their local multiplex. We all know how successful they've been (this is the highest grossing film series of all time) but does that mean we will be watching the Potter films 10 years from now? With the series finally ending, it is to be expected that commentators are asking how good the Potter films really were and what their lasting cultural significance is likely to be.

For some, the end of Potter can't come soon enough. After the eight films in 10 years, critics recently have been showing mounting signs of Harry Potter fatigue. It's instructive to compare the generally wildly enthusiastic response in British newspapers to Harry Potter and the Philosopher's Stone in 2001 to the far more muted reviews that greeted The Deathly Hallows: Part 1 last autumn.

"Richly accomplished entertainment spectacular, the quickest, zappiest two hours you'll spend in the cinema," one leading broadsheet writer enthused of the first film, but was far more grudging by the time of the first Deathly Hallows nine years later, grumbling about the "impenetrable plot quirks" and the "usual mythical baggage" that weighed down the film. "I have become resigned to the Harry Potter movies having only as much interest and power as one of the rides in the Wizarding World of Harry Potter theme park," the same reviewer wrote.

Throughout the last decade, Potter films have continued to be snubbed at the Oscars, winning occasional nominations in technical categories but never carrying off the main prizes.

Perhaps the arrival of the last film in the series will prompt a more generous critical and awards response. After all, Deathly Hallows: Part 2 marks the final showdown against Voldemort. It's the last hurrah of a franchise that has given the UK film industry a huge fillip (even if it was US-financed); that has made international stars out of young British actors like Daniel Radcliffe and Emma Watson and that has boosted everything from tourism to the profit margins of British post-production houses.

With worldwide box-office receipts that are bound to be well over $7bn, Warner Bros won't be fretting too much over the occasional lukewarm review or the absence of major awards. The Potter theme parks and JK Rowling's recently announced interactive website Pottermore will ensure that the cult of Hogwarts continues – as does its ability to generate revenue. The Harry Potter films may be over but the Harry Potter industry will continue apace. You'll still be able to buy the Potter Lego, tie-in toys, merchandise and, of course, to carry on reading the books. It will be intriguing to see how much licence Rowling allows Warner Bros. The US studio would surely relish the chance to make animated series, TV dramas and to extend the Harry Potter "brand" in the same way that George Lucas did with Star Wars.

The abiding interest in all things Potter is underlined by the intense curiosity displayed toward each new career move by Daniel Radcliffe and Emma Watson. This has occasionally been prurient and intrusive (for example, when Radcliffe had a nude scene on stage in Equus, or when it emerged that Watson had become one of the most highly paid actresses in the world). At the same, the goodwill of Potter fans toward the two stars has been evident as both have begun to succeed in carving careers for themselves away from Hogwarts.

Radcliffe has recently enjoyed a mini-triumph in a singing and dancing role in the Broadway revival of the play How To Succeed in Business Without Really Trying. The acerbic New York Times critic Ben Brantley gave him six out of 10, suggesting that his "effortful performance is sure to stir maternal instincts among women of all ages (and probably some men too) and comradely protectiveness among his fans". Other reviewers were generally kinder. Next year, when audiences are beginning to crave their next Potter fix, they'll be able to see him on screen in Victorian costume in Hammer's The Woman in Black (which is out in February.)

Meanwhile, we'll soon be able to see Emma Watson on screen in My Week With Marilyn, a new film about Marilyn Monroe's time in Britain in the late 1950s, when she was shooting The Prince and the Showgirl. Watson doesn't get to play Marilyn Monroe herself (that role has gone to Michelle Williams, while she is cast as a wardrobe assistant.) Nonetheless, the film should help audiences accept her in parts other than as Hermione.

Now, the industry is looking toward The Hunger Games, the new movie franchise from the kids' sci-fi novels by Suzanne Collins that has long been hyped as Potter's natural successor. The film already has its slot in the release schedule in March next year. It chronicles Katniss Everdeen's defiant struggle to survive in a future dystopia where The Capitol requires its 12 subjugated districts to atone for their one-time insurrection through the annual "Hunger Games," a fight-to-the-death event held in a fantastical man-made arena. Jennifer Lawrence (Oscar-nominated for Winter's Bone) has been cast as Everdeen and is set to become a huge star.

Directed by Gary Ross (of Seabiscuit fame), The Hunger Games is planned as a trilogy. The film-makers are trying very hard to enlist the support of the fans of the novel – a tactic that paid handsome dividends for Harry Potter. Earlier this year Ross made a big show of musing over the recommendations of 134 teenage school students from Lubbock in Texas who sent him letters advising him on how he should make the movie, as part of a reading-group exercise.

At this stage, it's impossible to predict whether The Hunger Games has any chance of becoming the "next Harry Potter." Then again, as Tanya Seghatchian points out, when Heyday Films and Warner Bros first embarked on their Potter adventure, "none of us expected it to have the impact that it had." Ask her just why she thinks the Potter films turned into such a phenomenon and she says: "the characters are so authentic, identifiable and universal." Most girls, she suggests, think they might be Hermione or have shreds of Hermione in them. Meanwhile, the boys identify with Harry.

"He is someone into whom you can put so many of your own thoughts and fears and anxieties. It's that combination of a rich and complex world that is fun, surprising and inventive, with a very simple set of characters and story dynamics propelling it forward. It's the perfect balance of tension, surprise and familiarity!"

'Harry Potter and the Deathly Hallows: Part 2 (3D)' has its premiere on Thursday and is released on 15 July

Wizard wheezes? Franchises to follow Potter

The Hunger Games

Lionsgate is hoping for a whopping 'Harry Potter'-sized hit with its adaptation of Suzanne Collins' best-selling sci-fi book. Jennifer Lawrence stars and rock star Lenny Kravitz has landed a supporting role. There is no British flavour to this franchise and its darkness and violence may count against it.

Release date 23 March 2012

The Hobbit

A decade ago, Peter Jackson's 'The Lord of the Rings' was going hammer and tongs with 'Harry Potter' at the box-office. Then came legal wrangling and a lengthy hiatus. Now, director Jackson is back at work on a two part adaptation of Tolkein's 'The Hobbit' with many of the same characters and cast members. The odds are that it will do monster business, especially with no competition this time round from the Hogwarts crowd.

Release date ('The Hobbit, Part 1') 14 December 2012

The Three Musketeers

The much-filmed Dumas novel is coming back to the screen in 3D and with a very youthful cast in a version directed by Paul WS Anderson. Logan Lerman (fresh from playing teenage hero Percy Jackson) is D'Artagnan and Juno Temple plays Queen Anne. The film will offer swordplay instead of magic, but might still appeal to Potter fans. There is plenty of scope for sequels here, too.

Release date 12 October 2011

Horrid Henry: The Movie

Arguably aimed at too young an audience to have the universality of appeal of 'Potter', this adaptation of Francesca Simon's stories about an unkempt and very mischievous kid could still blossom forth into a franchise if the first film works at the box-office. One hitch is that it will be going head to head with 'The Deathly Hallows Part 2' for the summer audience, so it'll be tough.