The forms have to be returned by today if investors want their shares put into the auction being conducted by brokers Cazenove at the start of trading in the shares on 21 April.
Members have also complained that they they have been unable to get through on the customer hotline set up by Alliance & Leicester, and fax facilities have become overloaded.
A spokesman for the society said yesterday that as the Royal Mail can only guarantee to deliver 98.8 per cent of mail posted, it was inevitable that some packs would have gone astray out of the 2.3 million posted last month. He also admitted that the customer hotline could have been busy at times, especially immediately after the Easter holiday.
Because of the tight deadline there is no possibility of extending the time limit for returning forms in time to participate in the free sale.
But A&L is anxious to reassure customers who want to keep their shares that they will not lose their entitlement if they have not returned their blue or orange forms by today's deadline.
They will still be able to claim their shares at any time, and to choose whether to accept the offer of a nominee account or the actual share certificate.