The department will begin allocating the three million plates, which were returned as of the end of March, once it exhausts all ZZ plates, said Secretary for Transport and Logistics Lam Sai-hung.

That was in a written reply to a question by Legislative Council member Nixie Lam Lam on when the license plates would be exhausted, given that the plates, run from "AA" to "ZZ" followed by a sequence of three or four numbers, now being allocated are already at the "YN" prefix.

Under department protocol, a registration number must be returned if a driver has not reserved it after a vehicle's registration is canceled, or if another set of plates has been assigned.

"Based on the current rate of allocation, it is projected that such a number of TVRMs [traditional vehicle registration marks] is sufficient to meet the demand for not less than 20 years in the future," Lam Sai-hung said, adding that there was no need for further legislation.

The returned plates will be reallocated in reverse alphabetical order, starting from the "WA" to the "WZ" series, then the "VA" to "VZ" series, and so on.

Nixie Lam also asked whether authorities would have to modify traffic management computer systems at car parks and tunnels, to which Lam said the department plans to retain their current alphanumeric format, given its supply of registration plates.

Special alphabetical prefixes are reserved for designated vehicles. AM is for government vehicles, LC for Legco vehicles and ZG for People's Liberation Army vehicles stationed in the SAR.

Drivers may also apply for Personalized Vehicle Registration Marks, or vanity plates, which are often auctioned off for millions of dollars.