Diana Inc: The long, strange, and highly profitable afterlife of the People's Princess

She would have been 40 today. Four years after her death the Princess of Wales is a £40m a year industry

We've had tearful interviews with members of her entourage, gushing tributes from her fans, and endless speculation about her private life. But, in the grand tradition of all great media bandwagons, we ain't seen nothing yet.

After weeks of soft-focus documentaries and glossy magazine spreads devoted to her memory, TV chiefs are planning yet more programmes about Diana, Princess of Wales. Producers are to unveil details of two films marking the forthcoming 20th anniversary of her wedding to the Prince of Wales. One will take a nostalgic look at the day itself from the perspectives of those who were there; the other will set it in the context of the social climate of its time.

She would have been 40 today had she not been killed in the Paris car crash which also claimed her lover Dodi Fayed nearly four years ago and the "industry" cashing in on her memory is worth an estimated £40m a year.

In the murky realm of mail order websites and back street souvenir shops, "Dianamania" is enjoying a renaissance. Opportunist manufacturers are still defying the pleas of her family and friends by flooding the souvenir market with all manner of kitsch cash-ins.

Among the more bizarre items currently changing hands are Diana wallpaper patterns, computer screen- savers and fridge magnets. And there are dolls, an army of them, dressed in miniature versions of her most photographed dresses.

Meanwhile, today sees the start of this summer's open season at the princess's family home, Althorp. The estate is hoping that the addition of three new exhibition rooms will boost the number of visitors "substantially" from last year's 120,000.

The renaissance in the fortunes of the Diana industry will silence those who, only a year ago, were forecasting its imminent collapse. A combination of declining admissions to Althorp and plummeting revenue for the fund set up to continue her charity work was seen by many as proof that the tide of emotion following her death had finally subsided.

Now, some experts believe that the "cult of Diana" is undergoing a subtle transformation from an explosion of mass hysteria into a longer-term movement similar to the tide of hero worship of deceased Hollywood idols like Elvis Presley.

The latest round of TV tributes to the princess started in May, when Channel 5 screened Diana: The Fairytale Princess, a one-hour documentary focusing on her troubled private life. Though pitted against much-hyped ITV reality show Survivor and BBC drama Holby City, it attracted 1.7 million viewers at its peak – more than that night's episode of Brookside.

Other recent programmes have proved equally popular. A fortnight ago, ITV's flagship current affairs show, Tonight with Trevor McDonald, carried an in-depth interview with the late princess's brother, Earl Spencer, under the title Diana, My Sister. Despite its late time slot, more than five million people tuned in.

Meanwhile, the network's exhaustive serialised portrait, Diana: Story of a Princess, ends tonight, after attracting nearly six million viewers for each of its four instalments. Now, not to be outdone, BBC1 and Channel 4 are competing for a slice of the action, with two rival documentaries focusing on Diana's marriage to Prince Charles.

Billed as a "nostalgic look" at the 1981 royal wedding, BBC Entertainment's You Had to Be There is expected to be shown on the anniversary itself, 29 July. Producers stress that it will focus on the recollections of "ordinary people", including police officers and choir singers involved in the event, but among those interviewed is David Emmanuel, the celebrity designer responsible for Diana's wedding dress.

For its part, Channel 4's factual department has set out to take a more sociological approach. With the aid of grainy news footage, Royals and Riots will attempt to draw a stark portrait of the social tensions that bedevilled the early years of Thatcher's Britain, by contrasting the glitz and glamour of the wedding with the horror of the Brixton riots.

Despite its more serious tone, the film also allows scope for the views of the same David Emmanuel, who will be heard describing Diana's big day: "It was virtually flash-bulb glamorous. She was like a movie star."

As viewers are treated to a televisual Dianafest, souvenir-hunters too are once more finding themselves spoilt for choice. Last week, the UK's two leading celebrity glossies, OK! and Hello!, carried huge colour photo spreads dedicated to the princess. If past experience is anything to go by, both can expect to see a temporary jump in sales of around 20,000.

Meanwhile, the internet remains a browsers' paradise for Diana fans. One website features a gallery of paintings by the American artist S Robert Lynch. Postcards based on the pictures can be ordered online for £26 a piece.

The Diana, Princess of Wales Memorial Fund, set up to raise money for causes close to the princess's heart, has spent the past four years fighting to prevent the exploitation of her name and image for commercial gain. While its overall annual income has taken a nosedive from around £94m a year at the height of Dianamania, in 1997/8, the fund still makes around £9m a year. At least £2m of this comes from sales of official merchandise, including Diana roses, tapestry cushion kits and its latest line, limited edition "Princess of Wales bouquets".

Dr Sandra Scott, a psychiatrist who has charted the evolution of the Diana phenomenon, believes that both the merchandise and the fan-base surrounding it are here to stay.

"Just as people commemorate the birthdays and anniversaries of their dead heroes, they are going to continue to do this, but with a greatly dwindling impact when compared with what happened when she first passed away," she says.