World health officials last night put out an unprecedented warning that deadly new strains of tuberculosis, virtually untreatable using the drugs currently available, appear to be spreading across the globe.

The new strains are known as extreme drug-resistant TB, or XDR-TB. They have been identified and have killed people in several countries, including the United States and eastern Europe, and they have recently been found in Africa, where they could swiftly put an end to all hope of containing the Aids pandemic through treatment.

Yesterday Paul Nunn, who heads the World Health Organisation's TB resistance team, said the situation was very serious. There are 9m cases of TB in the world and the WHO estimates that 2% of them - or 180,000 - could be XDR-TB.

"This is raising the spectre of something that we have been worried might happen for a decade - the possibility of virtually untreatable TB," said Dr Nunn.

Even in the United States, which has the best medicines available, a third of those who have been diagnosed with XDR-TB have died. In March, the Centres for Disease Control in the US registered that there had been 64 cases of XDR-TB; 21 of those ended in death.

Significant numbers of cases have been confirmed in Latvia and Russia, but in many parts of the world, XDR-TB could be rife but unrecognised. One of the reasons the WHO is concerned is that tuberculosis spreads easily in confined places, such as aircraft. Multi-drug resistant TB strains - those that are resistant to the two basic, first-line drugs used to treat the disease - have spread everywhere, including to the UK. Multi-drug resistant TB is increasingly common and is difficult and expensive to treat. The patient must be given four out of the six existing second-line drugs.

But the XDR-TB strains now appearing are a medical nightmare because at least three out of those six second-line drugs have no effect. There are no third-line drugs.

The spectre of a new untreatable plague has concentrated minds because of the identification of a cluster of cases in KwaZulu Natal, in South Africa. Scientists ran tests on people with tuberculosis in a rural part of the region. They studied 544 patients and found that 221 had TB strains against which the two common drugs, rifampicin and isoniazid, had no effect.

Finding such a high rate of multi-drug resistant TB was serious enough. But they also discovered that 53 of the patients had XDR-TB - and 52 of them died within an average of 25 days.

All the XDR-TB patients who could be tested were found to be HIV positive. Anybody with the virus becomes very susceptible to all types of infection. Tuberculosis is a major killer of people with Aids.

But the swift deaths of all but one of the study group in KwaZulu Natal has huge implications for the antiretroviral (ARV) drug treatment programme being rolled out across Africa in the hope of keeping millions of people with HIV alive and well, pending a cure.

"There is no point in investing hugely in ARV programmes if patients are going to die a few weeks later from extreme drug-resistant tuberculosis," said Dr Nunn.

The XDR-TB cases in South Africa were discovered only because Harvard scientists embarked on a study to gauge the extent of drug resistance. In other parts of Africa, there are no researchers or facilities to make the diagnosis, let alone monitor the numbers. "In most of the countries to the north of South Africa, they wouldn't be able to tell you what is going on," said Dr Nunn.

Most African countries either do not have a national reference laboratory for TB or they do not have enough. Kenya has one, he said; it should have four or five.

Tomorrow, WHO officials and international TB experts will take part in an emergency two-day meeting in Johannesburg, South Africa, to decide what action must be taken to address the crisis. They are expected to make recommendations that will include the importance of ensuring all TB patients take their full six-month course of drugs, to try to prevent resistance developing. In much of the world, that has proved difficult.

Tuberculosis drugs are old drugs. The white death or consumption, as TB used to be known, was thought to have been conquered more than 50 years ago. Drug companies have not invested in tuberculosis because it has been considered a disease of impoverished developing countries.

More recently, as the Aids pandemic has alerted the world to the inadequacies of medical treatment for a number of diseases in Africa, public-private partnerships have been been set up to attempt to find and develop new drugs, but there are none on the horizon yet.

Explainer: Tuberculosis

TB's origins can be traced back to antiquity, from the spinal cord fragments of Egyptian mummies to the writings of Hippocrates who identified phthisis, the Greek term for consumption, as the most widespread disease of the period.

Its symptoms - red swollen eyes, pale skin, coughing blood - led to it being labelled as vampirism during the Industrial Revolution. It was identified as a disease in the 1820s but was not named tuberculosis until 1839. Robert Koch's discovery of the microbe responsible, Mycobacterium tuberculosis, in 1882, earned him the Nobel prize in medicine.

In the early 19th century TB was rampant, responsible for up to one in four of all deaths in England. It spread through tiny particles released when an infected patient coughed or sneezed. Public health improvements cut the number of cases in the early 20th century, but not until the discovery of the antibiotic streptomycin in 1946 was the disease treatable.

Hopes of eradicating TB faded when drug-resistant strains emerged in the 1980s, followed by strains resistant to multiple drugs.