MARKETING text books everywhere will have to be rewritten. Marks & Spencer last week reported profits of more than £300m for the six months to September, but it also threw out a longstanding business principle that has served it well for many decades.

The profit performance is little changed on the same period last year.

That in itself could be regarded as a significant achievement in view of prevailing economic conditions.

Certainly, M&S executive chairman Sir Stuart Rose presented figures at the higher end of the scale predicted by many analysts.

Overall, in the period, food sales were up 1.8% – though they fell by 0.3% on a like-for-like basis in the half year.

M&S says it is “cautious” about prospects for the weeks ahead.

Like all other players in this sector though, it’s the next few weeks of trading, up to December 31, that will determine overall performance for 2009.

M&S is one of Britain’s strongest brands.

With that strength comes a level of brand equity others can only dream of.

And with the equity comes a shopping bag full of other strengths – customer loyalty, customer confidence, credibility, reliability – you think of one thing that matters to you as a consumer and chances are M&S ticks the box. The firm has provided a textbook example of how to create and develop a successful brand – a brand for which people feel immense loyalty.

And it is that very issue that has prompted the unprecedented decision of the M&S board to sanction stores stocking up to 400 products under their manufacturer’s brand.

So, soon you will be able to buy Coca Cola, Marmite or Baileys (to name but three) off the shelves of your previously rival brand-free M&S stores.

According to Sir Stuart, it’s what customers want. “They are saying to us… I like my favourite brand and you, Marks & Spencer, don’t sell it so I have to go elsewhere to buy it.”

That may be the reason why there’s to be such a fundamental change in a previously rigid policy. It’s understandable that the firm will seek to present the move as an enhanced service for customers, but could it also be that Sir Stuart sees such the year-on-year profit figure as stagnation?

Whatever the reason, the decision moves M&S into uncharted territory.