Locum pay across the UK has stagnated, and those working in London can receive as little as £150 a day, the 2009 Medeconomics survey reveals.
London locums have seen the lower end of their daily pay range halve in a year, according to the poll of 446 locums.
In 2008, the lowest day rate for London locums was £300. This had dropped to £150 this year.
The top end of the day rate range in London also fell from £625 last year to £530 in 2009.
One London GP said: 'Inner city London rates are considerably lower than other regions.'
The survey's highest earning locum was from the West Midlands and earned £168,000 per year. This compares with the top earner in London on £100,000.
GPC negotiator Dr Peter Holden said it was a matter of supply and demand in different parts of the county.
'London has always been over-stuffed with locums. They don't want to leave, which is fine, but it's a marketplace,' he said.
Dr Richard Fieldhouse, chief executive of the National Association of Sessional GPs, said regional differences in pay were to do with organisation rather than supply and demand.
Locums that organised themselves well as a group were likely to receive better rates, he said.
Over three-quarters of locums said their pay rates were unchanged from last year.
The survey found 40 per cent of respondents were working as locums because they had not been able to find a salaried GP post or become a partner.
One West Midlands doctor said: 'GP principal posts have more or less dried up.'
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