Millions of telephone numbers beginning with the prefix (020) 3 have now been issued, regulator Ofcom has announced.
Ofcom has released three million of the numbers to telecom companies
The numbers will begin to be allocated by telecoms providers later in the summer to residential and business customers in London wanting new lines.
The additional prefix has been introduced to meet the growing demand for new numbers in London.
There will be no change to (020) 8 and (020) 7 numbers which will still be issued until their allocation runs out.
Ofcom released three million of the new numbers to providers on Wednesday.
A further seven million will be released at a future date.
An Ofcom spokesman said: "This is the first time that the numbers are available outside of Ofcom.
"It is up to the service providers to decide when and how to dish them out."
Nearly 400,000 new numbers are allocated every year in London.
Ofcom says multiple occupancy flats and new businesses have driven up demand.
It believes the introduction of the (020) 3 would provide an initial 20 years' worth of new numbers.
The additional number range, the subject of an extensive advertising campaign in London and the south east, was first announced last July.
The (020) 7 and (020) 8 codes for Greater London were introduced during the Big Number change of 2000.
Ofcom has predicted that the stash of (020) 7 numbers will start running low by the end of the autumn.