More than 150,000 patients who were seen in London's Accident and Emergency units last year were not GP-registered at the time, the BBC has discovered.
Because many of them did not need emergency care, this put unnecessary pressure on A&E departments and staff.
One leading GP and author said doctors would struggle to cope if everybody in London tried to register with a GP.
Health Minister Ben Gummer admitted there were not yet enough GPs to meet the government's ambitions.
Responses to Freedom of Information (FoI) requests submitted by BBC Radio London showed 153,564 patients who presented at A&E in London in 2014 were not GP-registered.
The true figure is likely to be even higher, though, because not all NHS Trusts record the figures.
At Barts and the London hospital, 22,642 unregistered patients were seen, while 10% of attendees at Homerton in east London - which now employs Non-Clinical Navigators (NCNs) to help people sign up with a GP - were not registered.
NCN Faria Khattak said: "We tend to have a lot of fluid population, young professionals that move in and out of the area, so we get a lot of people [for whom] their first point of contact is coming to the hospital for care.
"We also get a fair share of immigrants who have no idea how the NHS works."
Health minister Ben Gummer told BBC Radio London: "We need more GPs and that's why the government at the election promised that we would deliver 5,000 more GPs over the course of this parliament, so by implication we don't have enough for the work we want to do.
"Part of which is delivering a seven-day NHS across the system and that is why we are promising to resource that promise with £10bn extra and part of that will be spent on additional GPs.
"So, we are responding precisely to the concerns that I know people have in London as they do across the country."
London-based GP Youssef El-Gingihy, who criticised the government in his book How To Dismantle The NHS In Ten Easy Steps, warned GPs could struggle with any increased demand.
"The proportion of the NHS budget going to general practice continues to drop at a time of rising demand," he said.
"There have been hundreds of GP closures and mergers across the country and a quarter of walk-in centres have closed since 2010."