Any restrictions on health and welfare benefits for immigrants from European Union states would have to apply equally to UK citizens under EU law, Downing Street has acknowledged.
Reports over the weekend suggested that changes could be rushed in before Bulgarian and Romanian citizens gain full rights to move to the UK at the end of the year, amid public concern about so-called "welfare tourism".
Ministers were said to be considering making immigrants wait for up to a year after settling in the UK before being able to seek hospital care including operations, though it is understood that emergency and ante-natal treatment would be excluded from the clampdown.
The proposed changes could form part of a drive to restrict immigrants' access to benefits, council homes and public services.
But it is understood that any restrictions would have to be based on residency, rather than nationality, in order to comply with EU laws - meaning that British nationals might also be required to prove their entitlement.
"We have to operate within the law, including EU single market law," the prime minister's official spokesman said on Monday.
"It is my understanding that one can't discriminate between EU nationalities within that law."
However, the spokesman poured cold water on suggestions that people might be required to show "entitlement cards" to prove their right to treatment or benefits, saying that the PM's opposition to ID cards remained "unchanged".
The spokesman said that a Cabinet sub-committee, chaired by David Cameron, had met "several" times to discuss the issue of migrants' access to benefits and that it was expected to come to a conclusion "in due course".
"There is an ongoing process of discussion of a range of options," the spokesman told a daily Westminster media briefing. "There is the usual process of considering options, but we are not at the announcement stage yet."
Asked what evidence the government had that welfare tourism was a problem, the spokesman replied: "I think there is a widespread public concern around the pressures around some services, be it housing, local authority services or the NHS.
"There is a widespread sense of concern and that is what the Government is considering how best to respond to."
TUC general secretary Frances O'Grady said: "Alarm bells should be ringing. The concern must be that the government is set to limit benefits and access to the NHS under the cover of Romanians and Bulgarians gaining the right to work here.
"The government must spell out its plans in full detail and come clean on which people living in the UK will lose out.
"Ministers must also clarify whether those, such as UK pensioners who have moved to Spain, will see their eligibility to health treatment affected too."