Christian Aid has expressed concern about the possibility of the UK Government cutting the international aid budget.
Chancellor Rishi Sunak is widely expected to announce a temporary suspension of the commitment to spend 0.7 per cent of national income on overseas aid in next week's spending review.
The charity says doing so would have a long-term impact on countries that need help the most.
Patrick Watt, Christian Aid's director of policy, public affairs and campaigns, said: "Cutting the aid budget - breaking a manifesto promise made less than a year ago - would have lasting consequences for people around the world, with up to 150 million individuals set to be pushed into extreme poverty amid the worst economic crisis since the 1930s.
"Nearly £3billion has already been cut this summer, making this move unnecessary. We must not have a cut on top of a cut.
He added: "The Covid-19 pandemic, which affects the poorest the most, offers a chance to build back with justice, meaning a sustainable and fair future for all. No one is safe until everyone is safe."
His comments come as nearly 200 charity leaders are calling on Boris Johnson to honour his commitments to the world's poorest people and maintain the UK's aid spending.
In the Commons on Thursday, the Prime Minister fuelled speculation, repeatedly sidestepping calls to commit to keeping the 0.7 per cent target - even though it was in last year's Conservative general election manifesto.
Sarah Champion, chairwoman of the Commons International Development Committee, asked the Prime Minister to "quash rumours and confirm his manifesto commitment" to 0.7% "now and going forwards".
The Prime Minister evaded the direct question, telling the Commons: "I think we can all be proud of our record on overseas aid and that will continue."
Mr Johnson was also challenged on the issue by Labour leader Sir Keir Starmer, who said breaking the 0.7 per cent commitment "will not only undermine public trust, but also hugely weaken us on the global stage".
The Prime Minister said the UK would "remain one of the biggest contributors to aid of any country on earth".
Former foreign secretary Jeremy Hunt urged the Prime Minister "not to listen to any voices in his ear" suggesting that cutting aid could help fund a £4 billion a year increase in defence spending announced by Mr Johnson.
In a letter signed by 185 development and humanitarian charity leaders including Save The Children, Greenpeace UK, Unicef UK, Friends Of The Earth and The National Federation of Women's Institutes, they urged Mr Johnson to rethink his plan.
"Now is not the time to renege on our promise to spend 0.7 per cent of our gross national income on aid and development," they said.
"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.
"A U-turn on your manifesto commitment to maintain the 0.7% target would signal we are a nation willing to balance its books on the backs of the world's most marginalised people, many of whom are dealing with the impact of Covid-19 on top of existing hardship."
Church leaders have also expressed their dismay over the controversy.
Rev Richard Teal, President of the Methodist Conference, and Carolyn Lawrence, Vice-President, said: "Times are tough for many of us – both here in the UK, and for our brothers and sisters across the globe. UK Aid is a percentage of the UK’s GNI – and so it changes each year, and is never beyond what we as a nation can afford.
"It is vital that we preserve the UK’s reputation as a champion of global justice. A measure of a nation’s values is seen not only by actions in times of plenty but by resolve during other times too.
"As Christians, we believe it is our duty to stand alongside our global neighbours, and seek justice for the most vulnerable. As we are called to love our neighbours – wherever they might be - remaining vocally committed to 0.7% GNI is a moral imperative."
Downing Street has said the Government remained committed to supporting the world's poorest people but acknowledged that officials were looking at how the aid budget was spent.