About 2.3 million people have left London since 1994, according to new research.
Halifax Financial Services says people left the city at a faster rate than they moved to it.
This has resulted in a population drop of 9% for London, based on figures from the Office of National Statistics.
The research contradicts a report released by London's mayor in 2002, in which he said the city is growing at an unprecedented rate.
Government predictions
In the report 'Planning for London's Growth', Ken Livingstone said London's growth is equivalent to it absorbing a city the size of Leeds and beats previous government predictions.
The report also stated that from 2000 to 2002 the London had gained about 190,000 more people.
But according to the Halifax, 440,000 people left London between 1999 and 2003, compared to 230,000 people who left during the previous five years.
The research found the majority of people who left the city were aged between 25 and 44, compared to the majority of people who moved to the city who were aged between 16 and 24.
Most of those leaving the city chose to move to the South East, followed by the South West and then the West Country.
The research also showed that the population of the West Midlands, North West, North East, Northern Ireland and Yorkshire and the Humber also declined.