A group of 165 global leaders has called for immediate and coordinated international action to tackle the twin health and economic emergencies caused by the Covid-19 pandemic.

Past and present politicians – including three former UK prime ministers – joined academics and civil society representatives to warn the G20 that the virus will return unless urgent action is taken to bolster health systems in poor countries of Africa and Latin America.

Amid concerns that the fight against Covid-19 is being hampered by a lack of united action by the body that represents leading developed and developing nations, the global leaders said $8bn (£6.4bn) of emergency health funding should be approved to prevent a second wave of infection.

A letter from the group urged the G20 to set up a taskforce with executive powers to help coordinate the fight against Covid-19.

It called for debt relief for the poorest countries and an increase in the funds available to the International Monetary Fund and the World Bank.

“We are writing to call for immediate internationally coordinated action – within the next few days – to address our deepening global health and economic crises from Covid-19” the letter says.

It adds: “In 2008-10, the immediate economic crisis could be surmounted when the economic fault line – under-capitalisation of the global banking system – was tackled. Now, however, the economic emergency will not be resolved until the health emergency is effectively addressed: the health emergency will not end simply by conquering the disease in one country alone, but by ensuring recovery from Covid-19 in all countries.”

The letter was signed by, among others, Gordon Brown, Tony Blair and John Major, the former president of the European Central Bank, Jean-Claude Trichet, the former Australian prime minister Kevin Rudd, the former Archbishop of Canterbury Rowan Williams, the former UN secretary general Ban Ki-Moon, financier George Soros and the nobel prize winning economist Joseph Stiglitz.

The letter said that in addition to the $8bn of emergency health spending, developing countries would need $150bn to fight the twin medical and economic challenges.

Both the IMF and the World Bank have been urging creditor countries to defer interest payments on bilateral poor-country debt this year, saving them $14bn, but the group of 165 leaders said the international community should go further and agree a more generous package, worth $44bn to Africa alone.

The IMF and the World Bank will hold spring meetings by teleconference, with finance ministers discussing plans to beef up the financial resources of the two Washington-based organisations.

The letter to the G20 said the IMF needed an additional $500-600bn in the form of special drawing rights: a form of international currency that can be used to increase the reserves of member states. Extra funds would also be needed to help the World Bank meet demands for help.

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Citing research by Imperial College London, the letter said 1.2 million Covid-19 deaths were possible in Africa and Asia’s poorest countries.

It said even the best funded health systems were struggling to cope with the pandemic. “Yet, if we do nothing as the disease spreads in poorer African, Asian and Latin American cities and in fragile communities which have little testing equipment, ventilators and medical supplies; and where social distancing and even washing hands are difficult to achieve, Covid-19 will persist there and re-emerge to hit the rest of the world with further rounds that will prolong the crisis.”

It added: “Much has been done by national governments to counter the downward slide of their economies. But a global economic problem requires a global economic response.”