The Leprosy Mission England and Wales has warned of a "Covid-19 human time bomb" in the poorest communities across Africa and South Asia.
Cases of coronavirus have been confirmed in all 10 countries where the charity works in these two regions.
They include Mozambique, where many people live without any access to healthcare. The entire country has just 30 ventilators for a population of nearly 30 million.
In the face of such a shortage, the Leprosy Mission fears that millions may needlessly die if coronavirus gains momentum in the country.
On the other side of the African continent, the spread of coronavirus threatens to undo the progress in fighting the deadly Ebola virus, which has killed over 11,000 people in West Africa in the last six years.
The Leprosy Mission warned that halting the spread of coronavirus in developing countries will be difficult because of low levels of literacy and digital communication, particularly in rural areas.
With many people living without running water in their homes, it says that it will be very hard for people to self-isolate and wash their hands frequently. In the face of this challenge, the Leprosy Mission is currently working on finding practical solutions to stop people congregating at public water pumps.
"Concepts such as social distancing are not only culturally alien but totally unattainable, for example in overcrowded slum communities. People are either unaware or very afraid," it said.
It went on to say that those living with leprosy are among the most at risk of Covid-19 because of their weakened immune systems.
Head of Programmes at Leprosy Mission, Sian Arulanantham, said: "We must act now, at a time when the coronavirus is on the brink of ripping through the majority world, overwhelming fragile health systems.
"Devastatingly, this is a human timebomb waiting to explode.
"As ever it is people living in leprosy-affected communities that will be hit the hardest. Well-off travellers have returned to developing nations bringing with them the Covid-19 virus. Yet these travellers are able to self-isolate, stockpile food and stay at home.
"Communities affected by leprosy are among the most vulnerable. Individuals are more likely to be disabled, have weakened immune systems and underlying health conditions.
"And like leprosy, malnutrition, overcrowded living conditions and poor sanitation make an ideal breeding ground for the spread of coronavirus."
The charity is working with partners across Asia and Africa to provide clear messaging about Covid-19 to leprosy sufferers and poor communities across the two regions.
In countries already under lockdown, it has created WhatsApp groups and is phoning people to share vital health information.
The Leprosy Mission is making preparations to support governments in testing and contact tracing of known cases to slow down the spread of the virus, but Ms Arulanantham warned that in densely populated areas, social distancing and isolation will be "an almost impossible task".
"These are communities who know only too well the suffering caused by fear and age-old myths, such as leprosy being a curse for sins committed in a previous life," she said.
"The truth is the greatest slayer of fear so it is vital that our communities, which all too often lie on the fringes of society, are reached.
"They must receive clear messaging on the symptoms of Covid-19 the importance of self-isolation, social distancing and the necessity for frequent handwashing."
With millions in poverty facing the prospect of even greater hardship, the Leprosy Mission is calling on governments to ensure that poor communities receive state aid, including food parcels.
The charity said it is preparing to provide practical assistance where this is not available from the state.
"People in the majority world face very different challenges to the Covid-19 pandemic from us in the UK," Ms Arulanantham explained.
"For those living hand to mouth, it is difficult to blame a day labourer for being tempted to work with a sore throat. If they stay at home, it is very likely their family will not eat that day.
"We heard of a young woman who cannot work because of lockdown and she and her family had no food for days. Her three children were crying with hunger. Thankfully we were able to help. We can't stand by and watch, we need to act."
Despite discrimination being illegal, Ms Arulanantham said that some people with leprosy scars are refused medical treatment.
"So, we are working hard to ensure anyone needing medical attention for symptoms of Covid-19 receives it," she added.