''I think that most of them were probably people who have moved overseas and never left a forwarding address,'' said Mr Bob Sherbin, a spokesman for the bank. ''Some of these people may also be deceased.'' Last week, the company had 805,068 shares from 1,573 shareholdings lodged in an account in Jersey, plus two years of accumulated dividends, and for anyone thinking that some of the shares might be theirs, there is a chilling warning: you have only 18 years left to claim them.

For 10 years after April 1991, the bank is required by statute to advertise every six months for shareholders to come forward with some proof that they own the shares, and identification.

A legal notice was placed in this newspaper and others late last week.

For the 10 years from April 2001, the bank is required to advertise annually.

In April 2011 the bank can keep any remaining shares, which would minutely improve some accounting ratios. Mr Sherbin points out that the shares it cannot trace represent less than 0.02 per cent of the issued share capital in Hongkong.

''Actually, I don't think we'd do that,'' said Mr Sherbin, although he admitted that it was difficult to guess the regulatory environment that Hongkong may be enjoying in 2011.

The procedure followed by the company is the same as that for any Hongkong listed company, but the pool of shares left in the company's hands is particularly large because of the bank's diverse long-term shareholder base.

Initially, the pool represented 1.2 million shares, worth $83 million at Friday's closing price of $69, and 1,875 shareholders.

After an initial surge of lost shareholders or their descendants coming forward, there has been a slow trickle of claims, which have to be addressed to the company's registrar, Central Registration.

Four old shares were replaced by one new share in HSBC Holdings two years ago as part of a move that appeared to indicate the company was establishing an escape route for 1997.

But, like every listed company, the bank knew already that some of its shareholders had disappeared, as previous dividend cheques or other shareholder mailings had been returned.

It is these shares that rest in an account with Coutts and Co in Jersey.

Incidentally, these passive investors have, unlike most retirement fund managers, outperformed the Hang Seng Index in the two years they have let HSBC hold their shares.

After the reorganisation HSBC Holdings had its first day of trade on April 8, 1991. HSBC shares have risen 194 per cent since then, against a rise in the Hang Seng Index of 167 per cent.