United's shares have crumbled steadily against the FT-SE index over the past 10 years, accelerated by the problems of Keebler, its arm in the US, this summer. Now it faces being booted out ofits FT-SE slot and its coveted place in index funds by BSkyB.
The threshold for FT-SE entrance has risen sharply from around pounds 1bn to more than pounds 1.5bn in recent years.
United, whose shares have dived by 80p this year, is in the relegation slot with a stock market value of pounds 1.51bn.
Close behind in the relegation stakes is Caradon, with a market value of pounds 1.55bn, Sears (pounds 1.61bn), MEPC (pounds 1.61bn), Cookson (pounds 1.8bn) and Tate & Lyle (pounds 1.81bn).
United has been a FT-SE stock since the Index started. It was a big enough company to be able to launch a pounds 1bn bid for the Imperial cigarette giant in 1987, though it lost the battle to Lord Hanson. In 1992 its shares touched 440p when rumours of a bid from Cadbury were running high.
There are still two and a half weeks to go before the axe falls. Pearson shareholders are gathering at an extraordinary general meeting to approve the BSkyB sale on 5 September and the Stock Exchange Committeewhich rules on FT-SE entrance, is due to meet the following morning.
United shares could yet be saved by a takeover bid or an early sale of Keebler, which is said to have attracted "a number of interested parties" with bids in the pounds 250-600m range, or just by a re-rating.
A stock exchange spokesman said yesterday that they will "go by the book" and relegate the company with the lowest stock market value at the close of business on 5 September.