Telephone numbers beginning with the prefix 020 3 are to be issued from next month, it has been announced.
Ofcom said telecoms providers will receive the numbers from 1 June. They will then be issued to residential and business customers from the autumn.
The communications watchdog said the additional prefix meets the growing demand for new numbers in London.
There will be no change to 020 8 and 020 7 numbers which will continue to be issued.
Ofcom first announced the additional number range for London in July 2004.
Ofcom's Matt Peacock, said: "We are introducing it because London is growing. There are more new businesses and households coming and we have to plan for the long-term.
"Bringing in these extra 020 3 numbers will give us an additional 20 years worth of new numbers for new Londoners."