Sales at Primark kept growing last year, but the share of each pound that turned into profit narrowed, and that shift is drawing attention as Associated British Foods (LSE:ABF) heads towards splitting off its retail arm.

Of all the numbers in the full-year results Associated British Foods (LSE:ABF) published on Tuesday 3 November 2026, the Primark adjusted Operating Margin may prove the most closely studied. It fell to 10.2% in the 52 weeks to 12 September 2026, down from 11.9% a year earlier.

The market's response to the results as a whole was negative. ABF ended the day 2.2% lower, having been down about 3% at midday, and was among the FTSE 100's notable fallers after forecasting lower annual profit. The Blue-Chip index itself dropped 1.14% to 10,638.43, its sharpest daily decline in a month according to Reuters.

An operating margin expresses operating profit as a proportion of revenue. If a retailer has a margin of 10%, it keeps roughly 10p of operating profit from every £1 of sales after paying for stock, staff, rent, distribution and other running costs, but before interest and tax.

Margins matter because they reveal something that sales growth alone can hide. A Business can sell more while earning less if costs rise faster than prices, if it discounts to shift stock, or if it invests heavily in new stores that have not yet matured. For a value retailer such as Primark, whose model depends on keeping prices low, even small movements in the margin can reflect meaningful changes in the balance between costs and pricing power.

Growing sales, thinner profits

Primark's Top Line held up. Sales rose 2% to £9.8bn. Underneath that headline, however, like-for-like sales, which strip out the effect of new or closed stores, fell 2.6%. That suggests the growth came largely from new space rather than from existing shops selling more.

The regional picture was mixed:

UK & Ireland:like-for-like sales rose 0.5%, an improvement on the 3.1% decline of the prior year. Primark's share of the UK clothing market also edged up to 6.8% from 6.6%.

Continental Europe:like-for-like sales dropped 4.7%, the weakest of the regions.

United States:sales grew 13%, although from a much smaller base than the European estate.

When sales at established stores slip while the cost of running them does not, the margin tends to come under pressure. That pattern helps explain why the profit side of the retail business weakened even as Revenue rose. ABF's Retail segment reported operating profit down 12% at £1,005m.

Primark also continued to spend on its future. Capital Expenditure at the retailer was £576m during the year, part of a wider group capex bill of £1.2bn. New stores, refurbishments and systems upgrades can weigh on margins in the short term because the costs arrive before the full revenue benefit.

One example of the forward programme is home delivery. ABF said Primark plans to launch UK home delivery by the end of calendar 2027, a significant change for a chain historically associated with in-store shopping only. Primark's chief executive Eoin Tonge framed the wider ambition as creating "a stronger, simpler, more scalable business."

Why the market is fixed on this figure

Several threads make the margin a focal point for FTSE 100 watchers.

First, it is the clearest guide to Primark's earning power as a standalone company. ABF has said the separation of Primark from its food businesses remains on track for December 2027. Once the two are apart, investors will be able to assess the retailer on its own numbers, and the margin will be a central measure of how profitable that business is.

Second, management has offered guidance on it. For the 2027 financial year, ABF expects Primark's margin to be around 10%, with new space expected to add about 4% to sales growth. In other words, the company is not signalling a quick return to the prior year's 11.9%. The guidance points instead to the margin settling close to its new, lower level.

Third, the margin interacts with the rest of the group outlook. ABF has said it expects group adjusted operating profit and adjusted Earnings Per Share to fall below 2026 levels, mainly because of its Sugar division, which is forecast to lose between £70m and £170m. With Sugar under pressure, the stability of Primark's profit contribution carries more weight in the overall picture.

For illustration only, consider what a margin difference means in pounds. On a hypothetical £1,000 of sales, a margin of 11.9% would produce £119 of operating profit, while a margin of 10.2% would produce £102. The gap is £17 for every £1,000 of sales. Scaled up across a business with billions of pounds of revenue, small percentage changes translate into large sums. This example uses round hypothetical figures and is not a reconstruction of Primark's actual accounts.

ABF's full-year numbers were softer across the board. Group revenue was £19.6bn, down 1% at constant currency, adjusted operating profit fell 14% to £1,508m and adjusted earnings per share slipped 11% to 156.0p. Return on average capital employed dropped to 12.7% from 15.5%.

Shareholders were offered continuity on the dividend, which was held at 63.0p for the year, but the company said it plans no further Buybacks at this stage so that both future businesses are well financed before the separation.

Key dates to watch

Thu 5 Nov:Bank of England MPC decision, with a 25bp rise to 4% widely expected; Next also publishes its Q3 trading update the same day, offering another read on UK clothing retail.

Thu 12 Nov:ONS first quarterly GDP estimate for July–September 2026 (7:00am), relevant for domestic consumer-facing stocks.

Wed 2 Dec:FTSE Russell December review announcement, after the UK close.

April–July 2027:ABF investor events across its food businesses, ahead of autumn Capital Markets Days for AB Foods and Primark.

December 2027:Planned Primark demerger from ABF's food business.

The bottom line

Primark's margin fell from 11.9% to 10.2% in a year when sales still grew, a combination that points to pressure at existing stores, particularly in Continental Europe, alongside continued Investment in expansion. Management's guidance for a margin of around 10% in 2027 suggests the lower level may be the new baseline rather than a temporary dip.

That is why the figure has become a reference point. As the December 2027 demerger approaches and Primark moves closer to standing on its own, its margin will be one of the most direct indicators of what the retailer earns from its sales. Updates on UK home delivery, the US expansion and Continental European trading will all feed into whether that number holds, rises or slips further.