Donald Trump's decision to block US funding for abortion services around the world could result in tens of thousands of women dying, with the decline of these services leading to millions of unsafe abortions in developing countries, aid organisations have warned.

Mr Trump signed an executive order reinstating a ban — known as the Mexico City Policy — which prohibits the US giving federal family planning funding to international health groups that provide abortion care. Shortly afterwards, the President's press secretary Sean Spicer said Mr Trump was “standing up for all Americans - including the unborn”.

But the policy is likely to hugely increase the rate of abortion in developing countries, causing the deaths of an estimated more than 21,000 women.

The ban, which was first implemented by Ronald Reagan in 1984 and has been enforced by all Republican administrations since then, has been called the “global gag rule” because it prevents medical professionals and aid workers whose work is supported by US grants from even discussing termination as an option for women with unwanted pregnancies.

The so-called 'gag rule' forces NGOs to choose between accepting family planning and being prohibited from providing abortion counselling, or refusing US family planning funds altogether and attempting to secure alternative sources of funding. This often means women and girls are prevented from accessing contraception and safe abortion consistent with the laws in their countries.

Mr Trump's move was condemned on Monday as an “assault on women’s health”, with activists warning the rule threatens to undermine the viability of countless groups that provide healthcare, including contraception and abortion services, to millions of women in the developing world.

Now it has emerged from reports in previous years that the policy is only likely to have the exact opposite effect to the one intended.

In pictures: Women of the world march against Trump Show all 32 1 /32 In pictures: Women of the world march against Trump In pictures: Women of the world march against Trump London An image of President Donald Trump is seen on a placard during the Women's March in London, England Getty In pictures: Women of the world march against Trump Sydney A view of the skywriting word reading 'Trump' as thousands rally in support of equal rights in Sydney, New South Wales EPA In pictures: Women of the world march against Trump Rome People shout and hold signs during a rally against US newly sworn-in President Donald Trump in Rome Getty Images In pictures: Women of the world march against Trump London A protester holds a placard during the Women's March in London, England Getty Images In pictures: Women of the world march against Trump Marseille A placard ready 'Pussy grabs back' is attached to the handle bar of a bike during a 'Women's March' organized by Feminist and human rights groups in solidarity with women marching in Washington and around the world for their rights and against the reactionary politics of the newly sworn-in US President Donald Trump, at the Old Port (Vieux Port) of Marseille, southern France Getty Images In pictures: Women of the world march against Trump Bangkok A young Thai girl holds a "women's rights are human rights" sign at Roadhouse BBQ restaurant where many of the Bangkok Womens March participants gathered in Bangkok, Thailand Getty Images In pictures: Women of the world march against Trump Bangkok A Thai woman takes a photo of a "hate is not great" sign at the women's solidarity gathering in Bangkok, Thailand Getty Images In pictures: Women of the world march against Trump Bangkok American expats and travellers gather with the international community in Bangkok at the Roadhouse BBQ restaurant to stand in solidarity in Bangkok, Thailand Getty Images In pictures: Women of the world march against Trump London Protetesters gather outside The US Embassy in Grosvenor Square ahead of the Women's March in London, England Getty Images In pictures: Women of the world march against Trump Marseille Women's March at the Old Port (Vieux Port) of Marseille, southern France Getty Images In pictures: Women of the world march against Trump Marseille Protestors hold placards reading 'My body my choice, my vote my voice' during a 'Women's March' organized by Feminist and human rights groups in solidarity with women marching in Washington and around the world for their rights and against the reactionary politics of the newly sworn-in US President Donald Trump, at the Old Port (Vieux Port) of Marseille, southern France Getty Images In pictures: Women of the world march against Trump Rome A person holds a sign during a rally against US newly sworn-in President Donald Trump in Rome Getty Images In pictures: Women of the world march against Trump Kolkata Activist Sarah Annay Williamson holds a placard and shouts slogan during the Women's March rally in Kolkata, India AP In pictures: Women of the world march against Trump Kolkata Activists participate in the Women's March rally in Kolkata, India AP In pictures: Women of the world march against Trump London A Women's March placards are rested on a bench outside the US Embassy in Grosvenor Square ahead of the Women's March in London, England Getty Images In pictures: Women of the world march against Trump London A women carries her placard ahead of the Women's March in London, England Getty Images In pictures: Women of the world march against Trump Manila Women protesters shout slogans while displaying placards during a rally in solidarity against the inauguration of President Donald Trump, in suburban Quezon city, northeast of Manila, Philippines AP In pictures: Women of the world march against Trump Berlin Protesters attend a 'Berlin Women's March on Washington' demonstration in front of the Brandenburg Gate in Berlin, Germany AP In pictures: Women of the world march against Trump Berlin Protesters attend a 'Berlin Women's March on Washington' demonstration in front of the Brandenburg Gate in Berlin, Germany AP In pictures: Women of the world march against Trump Berlin Protesters attend a 'Berlin Women's March on Washington' demonstration in front of the Brandenburg Gate in Berlin, Germany AP In pictures: Women of the world march against Trump Melbourne Protesters take part in the Melbourne rally to protest against the Trump Inauguration in Melbourne, Australia Getty Images In pictures: Women of the world march against Trump Macau Protesters take part in the Women's March rally in Macau Getty Images In pictures: Women of the world march against Trump Melbourne Womens march on Melbourne protestors marching during a rally where rights groups marched in solidarity with Americans to speak out against misogyny, bigotry and hatred Rex In pictures: Women of the world march against Trump Macau Protesters hold placards as they take part at the Women's March rally in Macau Getty Images In pictures: Women of the world march against Trump Macau Protesters hold placards as they take part at the Women's March rally in Macau, Macau. The Women's March originated in Washington DC but soon spread to be a global march calling on all concerned citizens to stand up for equality, diversity and inclusion and for women's rights to be recognised around the world as human rights Getty Images In pictures: Women of the world march against Trump Manila A mother carries her son as they join a rally in solidarity against the inauguration of President-elect Donald Trump as the 45th President of the United States in suburban Quezon city northeast of Manila, Philippines AP In pictures: Women of the world march against Trump Sydney An infant is held up at a demonstration against new U.S. President Donald Trump in Sydney, Australia Getty Images In pictures: Women of the world march against Trump Sydney A woman attends a demonstration against new U.S. President Donald Trump in Sydney, Australia Getty Images In pictures: Women of the world march against Trump Sydney A woman expresses her Anti-Trump views in Sydney, Australia Getty Images In pictures: Women of the world march against Trump Sydeney Protesters demonstrate against new U.S. President Donald Trump in Sydney, Australia. The marches in Australia were organised to show solidarity with those marching on Washington DC and around the world in defense of women's rights and human rights Getty In pictures: Women of the world march against Trump London Protesters march from The US Embassy in Grosvenor Square towards Trafalgar Square during the Women's March in London, England Getty In pictures: Women of the world march against Trump London Protesters carrying banners take part in the Women's March on London, as they stand in Trafalgar Square, in central London Reuters

A 2011 study found enforcement of the gag rule by the George W Bush administration led to an increase in abortion rates in sub-Saharan Africa, likely because of lost contraception access, while a 2010 study stated that it “hampered Ethiopia’s efforts to address high rates of unsafe abortion”.

Marie Stopes International, an NGO that lobbies in favour of abortion and provides a variety of family planning services in both the UK and abroad, warned women in developing countries would “pay the price” for Mr Trump’s reinstatement of the Mexico City Policy, saying it would result in thousands more maternal deaths every year.

Because of its belief that safe abortion is a vital component of women’s reproductive healthcare, the organisation will lose all funding from the United States Agency for International Development (USAID) so long as the Executive Order is in place.

It warned the loss of services that will result from Mr Trump's reinstatement of the ban during his first term in office could result in 6.5 million unintended pregnancies, 2.2 million abortions, 2.1 million unsafe abortions and 21,700 maternal deaths.

Marjorie Newman-Williams, Vice-President and Director of Marie Stopes International’s international operations, said: “All the medical evidence, as well as everything we know from our daily interactions with women, is unequivocal: if you take safe abortion services out of the reproductive healthcare package, it exposes women to risk.

“Every year, 21.6 million women are so desperate to end their pregnancy they put their lives on the line by risking an unsafe abortion. Thousands of them die and millions more are left with life altering injuries.

"Agreeing to the Mexico City Policy would mean accepting their fate and turning our backs on the very women who need us most.”

Trump Inauguration protests around the World Show all 14 1 /14 Trump Inauguration protests around the World Trump Inauguration protests around the World Activists from Greenpeace display a message reading "Mr President, walls divide. Build Bridges!" along the Berlin wall in Berlin on January 20, 2017 to coincide with the inauguration of Donald Trump as the 45th president of the United State Getty Trump Inauguration protests around the World An activist holds up a sign at the "We Stand United" rally on the eve of US President-elect Donald Trump's inauguration outside Trump International Hotel and Tower in New York on January 19, 2017 in New York Getty Trump Inauguration protests around the World Protesters burn a U.S. flag and a mock flag with pictures of U.S. President-elect Donald Trump outside the U.S. embassy in metro Manila, Philippines Getty Trump Inauguration protests around the World Filipino protestors hold placcards during a protest rally in front of the US embassy in Manila, Philippines, 20 January 2017. On the eve of President-elect Donald Trump's inaguration as the 45th president of the United States, Filipinos and Fil-Americans held a protest in front of the US embassy in Manila to denounce the incoming US president. Getty Trump Inauguration protests around the World Hong Kong police officers and security guards look on as an anarchist protester belonging to the Disrupt J20 movement sits after using a heavy duty D-lock and motorcycle lock to chain himself to a railing at the entrance gate to the Consulate General of the United States of America in Hong Kong to protest the inauguration of United States President-elect Donald Trump, Hong Kong, China, 20 January 2017. Two activists were arrested and taken away by Hong Kong police during the demonstration. Getty Trump Inauguration protests around the World A banner is unfurled on London's Tower Bridge, organised by Bridges Not Walls - a partnership between grassroots activists and campaigners working on a range of issues, formed in the wake of Donald Trump's election, which aims to build bridges to a world free from hatred and oppression. Getty Trump Inauguration protests around the World Protesters chain themselves to an entry point prior at the inauguration of U.S. President-elect Donald Trump in Washington, DC, U.S. Getty Trump Inauguration protests around the World Bridges Not Walls banner dropped from Molenbeek bridge in Brussels, Belgium, 20 January 2017, in an Greenpeace action part of protests Wolrd protest in solidarity with people in the US, the day Donald Trump sworn in as the 45th President of the United States. Getty Trump Inauguration protests around the World A woman holds an anti-U.S. President-elect Donald Trump placard during a rally in Tokyo, Japan, Getty Trump Inauguration protests around the World A Palestinian protester holds a placard during a demonstration against the construction of Jewish settlements in the occupied West Bank and against US President-elect Donald Trump, on January 20, 2017, near the settlement of Maale Adumim, east of Jerusalem Getty Trump Inauguration protests around the World Banners on North Bridge in Edinburgh as part of the Bridges Not Walls protest against US President Donald Trump on the day of his inauguration Getty Trump Inauguration protests around the World Russian artist Vasily Slonov (L) and his assistant carry a life-sized cutout, which is an artwork created by Slonov and titled "Siberian Inauguration", before its presentation on the occasion of the inauguration of U.S. President-elect Donald Trump, in a street in Krasnoyarsk, Russia Getty Trump Inauguration protests around the World A woman holds a banner during a march to thank outgoing President Barack Obama and reject US President-elect Donald Trump before his inauguration at a park in Tokyo, Japan, 20 January 2017. EPA Trump Inauguration protests around the World Palestinian demonstrators protesting this week against a promise by Donald Trump to re-locate the US embassy to Jerusalem Reuters

Another NGO working on abortion services in developing countries, the International Planned Parenthood Federation (IPPF), warned that the impact of Mr Trump's decision would be "devastating".

Ms Amu Singh Sijapati, President of Family Planning Association of Nepal, where the organisation runs a -large US-funded project to strengthen family planning service delivery systems within Government Health Ministries, said: “Our USAID-funded projects have been really successful.

"Already we have been able to train health workers, expand clinics and provide a much broader mix of contraceptives choices like long acting methods through satellite services that are targeted to really disadvantaged groups.