With the breakneck speed of expansion of cities, India has the greatest number of urban-dwellers in the world without safe, private toilets and practising open defecation though the government has made sanitation a priority, a report by a global NGO says.

"India ranks top for having the greatest number of urbanites living without a safe, private toilet – 157 million – as well as the most urban dwellers practising open defecation — 41 million," the report by WaterAid, a UK-based charity working in the field of safe water and sanitation, said.

The second annual analysis of world's toilets 'Overflowing Cities The State of the World's Toilets 2016', released on the eve of World Toilet Day being observed on Saturday this year, also says the high population density of urban areas means that diseases spread fast in the absence of good sanitation.

The report says the problem was so big that the daily waste produced on the streets of India's towns and cities was enough to fill eight Olympic-sized swimming pools, or 16 jumbo jets with poo every day.

Noting that the country was going through the largest rural-to-urban migration of this century, it said that Prime Minister Narendra Modi's Swachh Bharat (Clean India) campaign recognises the country's health and wealth largely depends on towns and cities growing in a sustainable, equitable way.

"In today's India, 381 million people – a population roughly the size of Western Europe – live in rapidly expanding urban areas, and 157 million of those people have nowhere decent to go to the toilet," it said.

Even though the government has made sanitation a priority, the numbers of urbanites living without sanitation has swelled by 26 million since year 2000 as cities expand at breakneck speed, the report, which examined the problems facing more than 700 million urban dwellers around the world living without decent sanitation, said.

An estimated 100 million of these have no choice but to defecate in the open — using roadsides, railway tracks and even plastic bags, dubbed 'flying toilets'.

WaterAid's Director of Programmes & Policy, Avinash Kumar, said India's rapid and unplanned urbanisation has resulted in various developmental challenges.

"... Current rates of urbanisation and meeting the increasing demand for basic services may make achieving Sustainable Development Goals a problem," he said.

Kumar said there was a vital need for an integrated approach towards urban planning that prioritises provision of basic services like clean water, safe sanitation and sustainable faecal sludge management by ensuring people's participation.

"This is the only way to create a healthier and more sustainable future," he said.

The and Smart Cities Mission are good government initiatives, but they need to take into account the ability of Indian cities to absorb and handle the projected population growth in urban centres, Kumar added.

The report examines the state of city sanitation around the world, an issue becoming more pressing as two-thirds of the global population are expected to live in towns and cities by 2050.

According to it, war-ravaged South Sudan, the world's newest nation, is the worst country in the world for urban sanitation in percentage terms.

An estimated 84 per cent of urbanites have no access to a toilet and every other urban-dweller there practises open defecation, it said.

Africa's biggest economy, Nigeria, is falling furthest behind in reaching its urban population with a toilet.

For every urban dweller reached with sanitation since 2000, two were added to the number living without, an increase of 31 million people in the past 15 years.

The report, however, says fast-growing China is making the most progress in reaching its urban population with sanitation.

"It's managed to build toilets faster than the pace of new arrivals, reaching 329 million people since 2000, and outpacing population growth by 9 million," it said.

The high population density of urban areas means that diseases spread fast in the absence of good sanitation.

One child dies every two minutes from diarrhoeal diseases caused by dirty water, poor sanitation and hygiene.

Globally 159 million children under five have their physical and cognitive development stunted; many of such cases are caused from repeated bouts of diarrhoea attributed to dirty water, poor sanitation and lack of hygiene.

The report suggested that everyone living in urban areas, including slums, to be reached with a toilet to ensure public health is protected.

It also suggested more money, better spent from governments and donors on sanitation, clean water and hygiene for the urban poor.

It stressed the need for coordination from all actors in the sanitation chain including governments, city planners, NGOs, the private sector, informal service providers and citizens.

The report also said sanitation workers to be given the respect they deserve with stable employment, safety and decent pay. Without them healthy communities and cities are impossible.