London relies more on overseas-trained health staff than any other part of the UK, says a report by the King's Fund.
The NHS could face US competition for its overseas recruits
In the three London NHS trusts studied by the health charity, between 12% and 25% of nurses were not UK trained.
The results are up to six times higher than Royal College of Nursing (RCN) figures, showing 4% of its UK members qualified abroad.
The report's author, Professor James Buchan, said London's NHS should recruit more from local communities.
He said: "International recruitment was initially regarded as a quick-fix, but it's now clear this is an integral part of many hospitals' recruitment strategies.
"While overseas health workers are playing a key role in delivering patient
care in the capital, the NHS cannot afford to rely too heavily on them."
'Acute staff shortage'
He said more local recruitment would provide "a more sustainable solution to London's health workforce challenges".
The RCN's Lucy Hamilton told BBC News Online the figures were no surprise.
She said: "There is a high turnover of nurses in London, we have an acute shortage of staff here and generally the NHS is being propped up by nurses from overseas."
The study also says increasing competition for overseas recruits from other countries could mean more problems for London.
King's Fund chief executive Niall Dixon said the policy should also be reassessed to ensure it is not depriving patients in developing countries of health professionals.
The NHS says its code of ethics prevents hospitals recruiting from certain developing countries that cannot afford to lose health staff.