Christian organisations are among the nearly 200 charities urging Boris Johnson not to slash Britain's overseas aid budget.
The Government is planning to reduce the international development budget from 0.7% to 0.5% of GDP, amounting to billions in lost aid for poorer countries.
Johnson has refused to commit to retaining the current aid budget in Parliament, despite 0.7% being a Tory manifesto promise, but has in the last week announced an additional £16.5bn for defence.
The open letter to Johnson has been signed by the chief executives of 187 charities, among them The Salvation Army, Tearfund, Christian Aid and CAFOD.
They say that some 115 million people are at risk of being "pushed back into extreme poverty" after months of Covid-19.
The letter argues that the pandemic calls for "an international, collaborative response" and that reducing Britain's development budget risks its reputation worldwide.
"Now is not the time to renege on our promise to spend 0.7% of our gross national income on aid and development," they write.
"Stepping back from our international commitments is not the solution and risks damaging the UK's standing globally as we define our role in the world post-Brexit."
With the UK due to host a G7 summit and the COP26 climate change summit next year, the charity heads conclude by saying that now is the time for "increased, not decreased, engagement from the British government in its efforts to make the world healthier, safer and more prosperous".
"This pandemic has shown us that no-one is safe from this virus until we are all safe," the letter reads.
"The UK's experience and credibility as a 'development superpower', rooted in its commitment and expertise, means it has a critical leadership role to play in helping strengthen global health systems and peacebuilding, tackle global challenges and reduce poverty to reduce the risks to us all from the next pandemic.
"Now is not the time to signal a withdrawal on the world stage by reneging on the Government's commitment."