Justin Urquhart Stewart, a director at Barclays Stockbrokers, says the bonus applies to the share issue offered for sale in December 1991.
When the shares were floated, successful subscribers were told that if they kept their holding for three years, they would receive one free BT share, up to a limit of 150, for each10 shares.
"Some brokers offer cheap share-dealing services and it may be tempting to sell off a few shares to help pay for the Christmas bills. But if shareholders do so, they lose out on the bonus, which belongs to the new owner of the shares," said Mr Urquhart Stewart.
"Shareholders will also be entitled to the dividend paid on the loyalty bonus shares themselves, due on 13 February `` Shares in the second BT offer were priced at 350p in December 1991, with members of the British public paying a discounted price of110pimmediately, followed by 120p in July 1992 and 105p in March 1993.
Since then, total dividend payouts have been 46.7p per share.
Mr Urqhart Stewart said: "The prospective yield on BT shares is 6 per cent. Whatever one's opinion of BT shares, their yield is one of the highest. Our advice is for people not to sell their BT shares but, if they must, at least to wait until the new share certificate comes out at the end of January."
He stressed he did not believe discount brokers would deliberately try to con people into selling shares.
But some customers of execution-only broking services might have forgotten the bonus offer in the rush to prepare for Christmas.