Hospitals will be required to check the immigration status of "suspicious" patients under a crackdown on foreigners getting free NHS operations that is due to be announced today by John Hutton, the health minister.

The plans come as the former cabinet minister Stephen Byers prepares to re-establish his credentials as an advocate for New Labour renewal with a wide-ranging call tomorrow for illegal immigrants to be denied access to schools and hospitals, in order to ease public concern.

Mr Hutton will promise that the tougher policy will not lead to discrimination against ethnic minorities. But health unions last night feared that the move would turn front-line NHS staff into a branch of the immigration service.

"Staff in hospitals have a duty of care, not a duty to police who have access to that system," said Karen Jennings, the national secretary for health at the public service union Unison.

The new policy will be set out in a consultation paper today. It will be presented as a necessary tightening up of the NHS principle that everyone is treated free of charge if they have been living in Britain for at least 12 months.

Mr Hutton will claim anecdotal evidence of failed asylum seekers, illegal immigrants and those overstaying temporary visas getting free treatment in NHS hospitals. The Department of Health describes this as "bizarre" generosity at the taxpayers' expense, although it has no estimate of the numbers of cases.

The consultation paper will propose removing the right to free treatment from anyone who does not have legitimate authority to reside in Britain.

Draft guidance to NHS trusts would require staff to check patients' residence status in cases where they have reason to doubt it.

A health department spokesman said the changes would not affect free access to GPs or to hospital casualty departments. Patients from other European Union countries would still be able to access the NHS under reciprocal arrangements for visitors.