Identity cards could create a "health underclass", the Liberal Democrats have warned as opponents of the scheme intensify their campaign.
The Home Office says it is still committed to ID cards
Lib Dem health spokesman Steve Webb said he feared people without ID cards could be prevented from seeing NHS doctors - except in emergencies.
The proposals could mean contagious diseases going undetected, he argued.
The Home Office says ID cards would just become the recognised way to check people were eligible for the NHS.
Critics of the planned ID card scheme wanted it scrapped after the minister responsible suggested its benefits had been "oversold".
Home Office Minister Tony McNulty, who is overseeing the scheme, told the Fabian Society meeting: "Perhaps in the past the government, in its enthusiasm, oversold the advantages of identity cards.
"We did suggest, or at least implied, that they might well be a panacea for identity fraud, for benefit fraud, terrorism, entitlement and access to public services."
Mr Webb said he was worried the government was talking about using ID cards to check entitlement for public services.
He has this week written to Health Secretary Patricia Hewitt asking her to address his concerns about the health needs of those without cards before proceeding with the scheme.
He highlighted the government has recent admission that there are perhaps 500,000 people in the UK illegally.
Those people were currently willing to go to the NHS for serious, but not life threatening conditions, said Mr Webb.
"The worry is that if the ID card turns into an entitlement card then increasingly they will not be able to access most NHS services," he said.
That could mean diseases such as TB went unchecked, he warned, or people would resort to a "black market in back street quackery" for healthcare.
Mr Webb said the problem was the Home Office's failure to locate illegal immigrants.
The humane answer to that problem was not to let those immigrants suffer in pain or go to "back street" doctors, he argued.
A Home Office spokesman said: "Emergency medical treatment or other services to do with genuine emergencies would never depend on production of an ID card."
Instead, the scheme would provide people with a more convenient way of showing their identity - something already needed to access public services, he said.