At St Mary’s Hospital in Paddington, London, the HIV-testing clinic is always busy and a cacophony of foreign voices sounds out from the waiting room.

Russians mingle with Africans, Brazilians with Romanians as they arrive for blood tests in order to ascertain whether they are suffering from what is Britain’s fastest- growing serious medical condition.

The remarkably swift and efficient NHS service at this world-famous hospital – and other HIV testing clinics across the country – is free to anyone, wherever they come from in the world.

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Around 7,000 people are diagnosed with HIV in the UK every year, and around 60 per cent are from Africa. Nigel Farage suggested yesterday that such people should be banned from Britain unless they have insurance

Those who test positive join the estimated 7,000 or so people in the UK who have been newly diagnosed every year over the last ten years as suffering from HIV.

Of those newly diagnosed in 2013, more than 60 per cent were born abroad.

Each HIV patient from overseas – whether here legally or illegally, whether a failed asylum seeker or student on a temporary visa – is entitled to free treatment on the NHS.

This is the result of a decision by the Coalition in 2012 to make HIV treatment free to all non-British visitors after lobbying by Left-leaning politicians and campaign groups. The aim was to stop the spread of the virus.

However, many believe that this change has led to an increase in so-called health tourism as foreigners deliberately come to this country to access free NHS treatment.

The process is simple: Foreigners can get a unique and permanent NHS number, which they are then usually able to use to get free hospital care. They can also book an appointment online or just walk in without a doctor’s referral.

Of course, that treatment is not actually free – it’s paid for by British taxpayers. The 2012 change in rules means that HIV sufferers can receive £20,000 of antiretroviral drugs every year.

In Australia, all immigrants have to pass a full medical test... an assessment is made about the likely impact of the costs of treating the person. If it is deemed too expensive, they can be refused entry

Once HIV treatment is started, patients will probably need to take the medication for the rest of their lives. In total, the cost to the NHS will be up to £1 million per patient if they survive into old age.

Not surprisingly, the policy of treating migrants with HIV has always been highly controversial. A new row was sparked on Thursday when Nigel Farage said that migrants with HIV should not be allowed to enter Britain without private medical insurance.

Although denounced by many, he won the support of Digby Jones, the former trade minister.

Lord Jones, an ex-head of the CBI who served in Gordon Brown’s government, said Mr Farage was ‘tapping into something important that people are beginning to say throughout Britain’.

‘We should be saying, “I don’t want you to come into this country with a communicable disease”.

But the row threatened to embarrass Douglas Carswell, Ukip’s new MP, whose father Wilson, an eminent doctor, helped diagnose some of the first cases of HIV in Uganda in the 1980s.

Earlier this year, 20 Tory backbenchers unsuccessfully tried to amend the Government’s Immigration Bill to make it law that before new immigrants enter the country, they must prove they are not HIV-positive and not suffering Hepatitis B – an equally devastating illness that is costly to treat.

Their bid was prompted by figures proving that most new cases of HIV in the UK are diagnosed in people who came here from abroad.

Britain’s open-door, free- treatment policy is very different to that operated by many other countries. According to the United Nations, almost 60 countries refuse entry to people who are HIV-positive.

In Australia, all immigrants have to pass a full medical test. Although an automatic ban is not imposed on those who test positive for HIV, an assessment is made about the likely impact of the costs of treating the person. If it is deemed too expensive, they can be refused entry.

The HIV unit at St Mary's hospital, Paddington, is free to anyone, wherever they come from in the world. If they are diagnosed, wherever they come from or however they got here, they are entitled to free NHS treatment

Meanwhile, doctors in Britain report increasing numbers of legal migrants and asylum seekers, particularly from Africa, who have HIV and other serious diseases. No one knows exactly what health tourists cost the Health Service each year.

Professor Meirion Thomas, an eminent consultant who has worked in the NHS for 44 years and who has researched the issue thoroughly, believes the cost to the taxpayer to be billions of pounds annually.

Some medical staff claim that migrants with HIV specifically target Britain because, unlike other countries, they can guarantee themselves a lifetime of free drugs and medical care, courtesy of the NHS.

As a result, taxpayers face a huge and ever-increasing burden because of the growing influx of migrants from countries where complex infectious diseases — such as HIV and tuberculosis — are common. Roger Williams, professor of hepatology at University College London, first sounded the warning some years ago.

He said: ‘The potential cost (of treating migrants) to the NHS is enormous in both financial and staff terms.

‘It is absolutely essential that firm action be taken to prevent any further increase in this growing pool of potential infection. Even a fraction of the money spent in the UK would treat a far larger number of patients in their own countries.’

Some medical staff claim that migrants with HIV specifically target Britain because, unlike other countries, they can guarantee themselves a lifetime of free drugs and medical care

His words have since been echoed by Professor Thomas, who says ‘health tourists’ come to the UK with pre-existing illnesses with the sole purpose of accessing free NHS care, and that our health service is being ‘bled dry’ by people suffering serious diseases such as HIV that require lengthy and expensive treatment.

He says patients often travel to Britain after being diagnosed with a serious illness in their home countries, where there may be poor or unreliable medical services or where the best care is expensive.

Professor Thomas adds: ‘There are tales of families relocating because a child has a severe congenital or acquired illness, of large numbers of patients with HIV coming to the UK because that is their only hope of getting effective treatment.’

Others disagree. For example, the National Aids Trust says there is no evidence of significant numbers of migrants arriving with the main purpose of getting free healthcare for HIV.

Its former chief executive, Deborah Jack, argues: ‘Leaving HIV untreated endangers public health by increasing the risk of transmission. HIV treatment is extremely cost-effective.

‘Not to provide it is the ultimate false economy, simply resulting in vastly inflated bills to the NHS later when people arrive in accident and emergency departments seriously ill after not getting early treatment.’

Whatever the arguments, the fact is that care is very costly and takes a long time.

Also, once here, it is hard to remove any migrant or asylum seeker with HIV or any other life-threatening ailment.

This is because of the way in which the NHS generously operates and because failed asylum seekers from countries with poor healthcare systems can claim that returning home would condemn them to a ‘certain death’.

They often go to court to fight any decision to repatriate them, on the grounds it would breach their human rights.

Britain has proud record of giving sanctuary to refugees – whatever problems they may bring with them.