A TB time bomb could explode on the world without major efforts to curb drug-resistant strains of tuberculosis, experts warned today.

No-one knows the full extent of the spread of resistant TB.

But two doctors writing in The Lancet medical journal argue that the "superbug" strains could become dominant without significant global investment to tackle the problem.

And they warn that if not enough is done potentially unstoppable forms of TB immune to all treatments could emerge in the future.

TB, once the scourge of the western world, still causes an estimated 1.8 million deaths each year.

Recently the infection underwent a resurgence, partly thanks to resistant strains developing as a result of poor treatment programmes.

In 2008, an estimated 440,000 multidrug-resistant tuberculosis (MDR TB) infections occurred around the world, accounting for 3.6% of all known cases that year.

However, only 7% of these cases were identified and treated, according to Dr Neel Gandhi, from Yeshiva University in New York and Dr Paul Nunn, from the World Health Organisation in Geneva.

An even more virulent strain known as extensively drug-resistant TB (XDR TB) had also been reported in most countries.

Although both strains are known to exist on every continent, the true extent of the threat remains unknown, said the experts.

Drug resistance surveys were carried out in only 59% of countries, and of these 20% relied on out-dated information collected before 2003.

It was also unclear whether drug resistance was increasing or decreasing since more than one survey had been completed in only 31% of countries.

India and China together carried nearly 50% of the global MDR TB burden, followed by Russia (9%).

Dr Gandhi and Dr Nunn wrote: "Unless countries invest substantially in management of MDR TB, the possibility remains that MDR strains could become the dominant form of TB. Moreover, the future possibility of strains that are totally resistant to all anti-tuberculosis drugs is not inconceivable."

Currently a handful of drugs are still available that can be used to treat MDR TB and XDR TB.

MDR TB is much harder to control than ordinary drug-susceptible TB. It requires 24 months of treatment instead of the normal six months, and costs £2,369 per patient compared with just £13.50.