Some institutional shareholders cashed in at the peak, selling their stakes even though they had pledged to remain long-term shareholders.
Last night’s 455p sale was also at a lower price than a 15% stake sale in June, which achieved 500p a share.
In trading this morning, shares slipped 4.5% to 450.6p.
The Government argues that there is no need for it to hold shares in Royal Mail as the universal postal services remains well protected by law and by the regulator Ofcom.