The trust is understood to intend to use the money to swell its charitable grants programme. A third of the proceeds will be used to fund a new science lab at Cambridge University.
City traders have taken the sale as a sign that a bid offer for the retailer is not on the horizon.
Seymour Pierce analyst Richard Ratner said: 'It confirms that there is a realisation among members of the family that they have made an awful mess of the past 15 years and that the chances of the shares, either on trading grounds or in a bid situation, being worth much more than the current share price is unlikely. We do not think a takeover bid where the offerer pays much more than 280p a share can work.'