Knickers, it seems, are not enough to keep M&S in the FTSE100 Index. Despite selling more than 60,000,000 pairs of pants in 2015, the High Street giant founded in 1884 by Polish-Jewish refugee Michael Marks is to be demoted from the FTSE 100 on September 23 after the company’s market value fell below the threshold for membership.

This painful blow is all the worse as M&S was a founding member of the prestigious City index. It’s a bit like Stephen Fry being kicked out of the Groucho Club.

Last week’s announcement led Times columnist Deborah Ross to urge readers to “save our Marks.” Otherwise, she averred with heavy irony, they risked “ending up wearing better clothes at better prices, better made from better fabrics…” bought from retailers such as Primark, Gap, Uniqlo and The White Company.

Where has M&S gone so wrong that it is now considered inferior to many High Street brands?

In its heyday in the 1980s, initially under the chairmanship of Marcus Seiff, progeny of one of the founding families (the Marks, Sachers and Sieffs who led M&S to the High Street dominance it enjoyed for many decades), and then of Richard Greenbury, M&S made chain-store shopping acceptable to the middle classes.

M&S was the go-to store for Italian-made suits, Jermyn-Street-quality shirts, on-trend, top-quality women’s clothing and La Perla-like lingerie. Fashion editors stalked the Marble Arch store in search of the latest catwalk look as recreated by M&S.

Poor M&S has never quite recaptured those heady days, though it has tried various tactics. It tried slashing prices to compete with cheap chains but if there’s the slightest change in a bra fastening or in the stretchiness of underpants’ elastic, the customers scream blue murder. There’s a perception that super cheap chains offer excellent quality. More likely, expectations are lower, as that chain’s reputation is not built on quality. Plus we live in a throw-away age, so if a skirt fades in a “delicate” wash, we just bin it.

They’ve tried celebrity endorsement. We’ve had Holly Willoughby, Twiggy, Rosie Huntingdon-Whiteley, Alexa Chung and David Gandy, and, of course, the Annie Liebovitz “Leading Ladies” campaign featuring, among others, Helen Mirren. Celebrity endorsement was part of a broader strategy to appeal to a more fashion-forward market as well as retaining the middle-aged and middle-of-the-road. But the approach alienated the core customer without capturing a younger crowd. When will M&S learn that whatever our age, we simply want beautiful clothes in beautiful fabrics.

They’ve also tried making their displays more appealing in line with those clever, closet-like sections seen in Zara and many European brands. But this effort is defeated by the utterly baffling “collections.” Why is there Per Una and Autograph? Some brands have “stories”; M&S has the never-ending story. Or it feels that way due to all these “collections” and to the sheer volume of merchandise.

Browsing is like being a castaway on a desert island, so few humans are around. Online shopping has obviously played a part in the reduced footfall, but the drop in customer numbers predates online shopping by several decades.

If I were chairman Archie Norman or CEO Steve Rowe, I’d look back at the 1980s. I’d produce fashionable, well-made clothes in good fabrics and charge a fair price. Then I’d trust the customer to judge the value of the offering. And I’d trust the customer to decide how to style it.

Jan Shure has been lambasting and defending M&S fashion since 1983.