An almost $1 billion shortfall in funding for the fight against polio is putting global efforts to eradicate the crippling viral disease in jeopardy, global health experts said on Wednesday.

In a report released 10 years after Europe was declared polio-free, the independent monitoring board of the Global Polio Eradication Initiative (GPEI) said there is now "a unique window of opportunity to stamp out polio for good" with global case numbers at their lowest levels since records began.

"But it will not happen if the programme remains so desperately under-financed," Liam Donaldson, the independent board's chairman, told reporters.

A disease that until the 1950s crippled thousands of people every year in rich nations, polio is a virus that attacks the nervous system and can cause irreversible paralysis within hours of infection.

It remains endemic in three countries - Pakistan, Afghanistan and Nigeria - after India in January became the latest country to become polio-free, after going a full year without registering a new case.

Polio often spreads in areas with poor sanitation - a factor that has helped it keep a grip on those endemic countries for many decades - and children under five are the most vulnerable.

The disease can be halted, as it was in Europe, with comprehensive, population-wide vaccination programmes.

Donaldson said the global financial crisis coupled with growing needs for polio funding had led to a shortfall of $945 million out of a total 2012-13 eradication programme budget of $2 billion.

The shortfall would have a "real and immediate" impact, he said. Vaccination campaigns for 2012 would have to be cancelled in 33 countries, leaving 94 million children "exposed to the horrors of this disease".

"This is an extreme and unacceptable risk," Donaldson said. "These campaigns are not optional extras, they are totally essential. It could set this global endeavour back by many years and vastly increase the eventual cost of achieving eradication."

Hailing progress made by India, which has now had 16 straight months without a single case of polio, Donaldson said that country should serve as an example for critics who have long doubted polio can ever be wiped out.

"Sceptics have always said that India would be the (eradication) programme's downfall...that it simply could not be done," he said. "It's clear that those people were wrong."

Donaldson's report, entitled Every Missed Child, focuses on the need to reach children who repeatedly slip through the net during polio vaccination campaigns.

It said that in six countries where polio persists - the three endemic countries plus Angola, Chad and the Democratic Republic of Congo - there are 2.7 million children who have never received even a single dose of polio vaccine.

"These are the children that the world forgot, growing up with no protection from polio at a time when most of the world's parents have long forgotten what polio even is," Donaldson said.

The report said the polio virus is hiding out in a number of "sanctuaries" - specific areas in the worst-hit countries where too many children have been missed in vaccination campaigns and where the virus can multiply and prepare for a fresh attack.

Donaldson, speaking to reporters in a telephone briefing from London, said wealthy donor governments needed to be aware that the campaign to wipe out polio was "at a tipping point" and urged them not to miss the opportunity to end polio.

"If the funding gap isn't bridged, future generations will surely look back and wonder how this once-in-a-generation opportunity to destroy polio...was allowed to slip through our fingers," he said. – Reuters