By Emma Birchley, Sky News Correspondent
Illegal immigrants suffering ill health are failing to seek treatment over fears that it could alert the authorities to their presence in the UK.
A new report has revealed that 90% of undocumented migrants are not registered with a GP and a fifth of those - more than 100,000 - say it is because they are too scared to come forward.
Richard is an asylum seeker and understands that anxiety. He came to Britain in 2003 fleeing persecution in Uganda.
"You get frightened so what happens is you decide to hide away and if you have any medical condition, any problem, you end up getting worse and worse and maybe in the end you may even die because you do not have any help," he said.
The concern outlined in the report by the think-tank Demos, due to be published on Monday, is that failing to detect ill health early can ultimately result in increased treatment costs.
But according to the document's author, Max Wind-Cowie, there is also the fear that transmittable diseases are being passed on to the wider public by those too anxious to seek help.
"When you think about global crises like ebola, what we need to make sure we do is find people who are unwell, make sure we are isolating them, make sure we are treating them," he said.
"We can't do that if they wait until they are incredibly unwell before going to seek health care because that means that it's already too late to prevent the infection from spreading."
In the end Richard did seek treatment from the organisation Doctors of the World in Bethnal Green, east London.
It is the kind of clinic that has been proposed in the report - an all-in-one centre where patients can see a doctor but general advice and legal support is also provided.
The difficulty is convincing vulnerable people it is safe to come forward.
One idea is to allow health professionals to access information from the Home Office without permitting data to flow the other way.
It will not be a popular plan with those who believe undocumented migrants should not have the right to free treatment.
But Dr Clare Shortall, who volunteers for Doctors of the World, welcomes any move to make sure those in need get help.
"Regardless of your position on immigration I think we are talking about vulnerable people who are victims of trafficking, children, victims of domestic violence, pregnant women. We are talking about them not being able to access health care."
