By Nick Triggle
Health reporter, BBC News
The system to ensure illegal immigrants do not receive free NHS care is unworkable, doctors and lawyers say.
The government says illegal immigrants should not get most NHS care
The government says failed asylum seekers or others without permission to be in the UK should only get limited free care, including emergency aid.
But GPs and primary care trusts said there was no way of ensuring illegal immigrants did not get free care.
David Lock, of law firm Mills and Reeve, which is advising some of the trusts, said the system was in a mess.
Mr Lock, a former Labour MP, said his firm represented scores of NHS primary care trusts and he was frequently being asked for advice.
He said he was telling them to carry on treating illegal immigrants who were already on GP lists as there was no practical way of removing them from the NHS register.
"The system is a complete mess. There is conflicting advice out there and NHS trusts do not know where they stand.
"If it is the case that illegal immigrants should not be getting free care, how is that to be enforced?
"It is a very complex situation, but the government needs to clarify its position. Trusts are quite rightly considering what their obligations are at a time when there are financial pressures in the NHS."
There are estimated to be about 400,000 illegal immigrants in the country.
Asylum-seekers are entitled to NHS care while their application is being processed.
But, if their claim is rejected, the government says they should not get the care free.
The same rules apply for people who have overstayed their visa or entered the country illegally.
There are a few exemptions for both GPs and hospitals relating to emergency care and immediate necessary care - to stop a condition becoming life-threatening.
Treatment for communicable disease such as TB can also be carried out free because of the potential impact on public health. On-going treatment is also often exempt.
A Department of Health spokesman said it was working to clarify the rules regarding access to NHS primary medical care.
He said: "We have undertaken a public consultation and ministers are considering the results and issues which it raised.
"The consultation's proposals were to align the rules around primary services more closely with those around hospital charges."
However, on the ground the reality is far from straightforward.
North Manchester PCT said it was the responsibility of hospitals and GPs to check eligibility, but they would not automatically rule someone out because of their immigration status.
Waltham Forest PCT said doctors checked eligibility when patients registered, but conceded that if their status changed - for example, their asylum application was rejected - it would be hard to pick this up.
Sheila Webb, director of public health in Bradford, said any checks that are made by GPs are limited.
"Patients would be asked for an address, but no checks are made to verify this. There is a confidential relationship that exists between a GP and their patients."
Doctors said illegal immigrants who were referred to hospital for treatment were sometimes spotted. In hospitals, a dedicated manager should assess someone's status and decide if they should go on a waiting list for an operation.
GPs said it was not their role to police the system and it was probable that illegal immigrants were being treated.
Dr Fay Wilson, a Birmingham GP and member of the British Medical Association's GPs committee, said: "In reality, the system is not working.
"NHS trusts have started asking us to carry out more checks, but GPs are not going to do it.
"The most sensible approach would be to bring back some form of national medical entitlement card. Then people could easily be checked."
Gill Morgan, chief executive of the NHS Confederation, said the system was complex, which meant identifying an individual's status was difficult and meant there "may be variation of interpretation across the country".
Sandra Gidley, Liberal Democrat health spokeswoman, said: "The government has failed to produce evidence that those who are ineligible for free care are draining the NHS's resources.
"We must be wary of scapegoating some of the most vulnerable people and fuelling prejudice."