The growing number of "health tourists" using the NHS is costing the taxpayer billions of pounds every year, a senior doctor has warned.
Maternity health tourism is a "massive and escalating problem", Professor J Meirion Thomas said.
The health service, which is in the middle of a £20 billion efficiency drive, is being used as the "world's maternity wing", said Prof Thomas.
Writing in The Spectator magazine, he said: "Foreign women often arrive in the UK in late pregnancy, often after detecting a complication. They come on a visitor's visa and present to A&E while in labour. Often the patient refuses to pay, claiming that a childbirth qualifies as emergency care and therefore cannot be refused to anyone. In this way, the NHS can be used as the world's maternity wing."
The cancer specialist added that there is "much evidence" of identity fraud, where expectant mothers give a registered name, address and NHS number - but are found to have different blood groups from the one on record.
He said there are similar "abuses" in oncology, HIV, infertility and in the treatment of renal failure.
Prof Thomas - a consultant surgeon at the Royal Marsden Hospital London, who holds a chair in surgical oncology at Imperial College - said he has heard "scandalous" reports of NHS abuse from numerous overseas visitor officers - who are employed by hospitals to identify, interview, invoice and recover costs from individuals.
"This abuse may be costing the NHS (and therefore the British taxpayer) not millions but billions of pounds every year," he wrote. "Curbing the abuse is not just right in itself, but may save astonishing amounts - and it can be done through a few simple procedures."
He urged Ministers to stop giving NHS numbers to overseas visitors adding: "Those who work and pay tax here, and have a National Insurance number to prove it, ought to be granted full access to the NHS whatever their residency status. But it makes no sense to accord this right to visitors, especially when an NHS number can translate seamlessly into expensive long-term hospital care."
Prof Thomas added that GPs should be instructed to make sure patients are eligible for hospital treatment before they refer them on. He also called for an audit into the issue.