Government claims that non-residents are costing the NHS as much as £2 billion a year were rubbished yesterday by doctors and opposition leaders.

Health Secretary Jeremy Hunt seized on the headline-grabbing report, released on the same day as the Con-Dem Immigration Bill had its first reading in Parliament, to claim that the NHS could legally be able to claw back at least £500 million of that amount from short-term migrants and foreign visitors who use the service.

The report estimates that so-called "health tourists" - people who travel to Britain specifically to use its health service - could cost as little as £10m.

Ex-pats coming home for free treatment are said to cost the NHS tens of millions a year.

But the report authors repeatedly point out the fragility of their calculations, warning: "The depth and completeness of our analysis has been constrained by the time limitations as well as limitations on the data."

It warns separately in a "note of caution" that its statistics "are based on incomplete data, sometimes of varying quality, and a large number of assumptions."

And British Medical Association GP committee chair Dr Chaand Nagpaul said: "There is limited evidence to suggest that migrants or short-term visitors are consuming large parts of the NHS budget.

"The government's estimates are based on a number of assumptions that result in a figure significantly higher than previous estimates."

Labour shadow health secretary Andy Burnham MP said: "It's hard not to conclude that this announcement is more about spin than substance.

"The government's own report undermines their headline-grabbing figures, admitting they are based on old and incomplete data.

"Labour would not support changes that make doctors and nurses surrogate immigration officials."

A recent European Commission report also concluded that so-called benefits tourism was "neither widespread nor systematic."

Movement Against Xenophobia's Guy Taylor branded yesterday's release a political stunt aimed at strengthening the government's anti-foreigner agenda.