David Cameron has defended his decision to spend 0.7% of national income on overseas aid as prime minister, calling it "an investment in our own security".
The former Tory prime minister said the debate should not be on whether money is spent on aid - but how it is spent.
Mr Cameron made the comments in an article for the Guardian, in which he revealed he is to chair a high-profile commission on overseas development.
The group will research fragile and conflict situations globally.
Spending on foreign aid rose to about £12bn when Mr Cameron was prime minister, and, writing in the Guardian, he said: "My view is clear: aid works."
International Development Secretary Priti Patel has warned that major multinational aid funding may be cut unless it provides better value for money, although the government remains committed to the 0.7% target.
To address concerns about value for money in aid spending at a time of austerity, ministers want to spend more of the money not only relieving poverty but also promoting Britain's strategic interests.
BBC diplomatic correspondent James Landale said this £700m "empowerment fund" could be used by the Foreign Office and other departments to help developing countries counter the threat of Islamic extremism or the clout of Russia.
In his article, Mr Cameron said that between 2011 and 2015, Britain had helped vaccinate 67 million children, saving at least 1.2 million lives from diseases such as pneumonia and diarrhoea.
"Such financial help doesn't benefit just the countries that receive that aid: money spent on international development is an investment in our own security," he said.
"Because if we don't tackle poverty abroad, the results are visited upon us at home.
"Mass migration, epidemics such as Ebola, climate change and pollution - none of these things respect national borders."
Mr Cameron, who stepped down as prime minister after the UK voted to leave the EU last summer, said he had decided to become unpaid chairman of the new Commission on State Fragility, Growth and Development, with Oxford University and the London School of Economics, to research what made a state fragile.
"As prime minister, I made sure that half Britain's aid spending went to the most fragile states," he said.
"Today, state failure is increasing and nearly half the world's poor people will soon live in fragile states and regions."
His commission, which will be co-chaired by Donald Kaberuka, the special envoy of the African Union Peace Fund, and Adnan Khan, the International Growth Centre's research and policy director, will be joined by eight commissioners from around the world to address what makes states fragile and keeps them that way.
The group will take evidence from witnesses ranging from military commanders in Afghanistan to aid workers in Syria, in the hope of "generating the most cutting-edge recommendations that governments, donors and NGOs (non government organisations) can put into practice".
This includes looking at the role small, medium and big companies can play - and how governments can create a tax base so they can pay for their own development.
"From defeating fascism and communism to spending 0.7% of our national income on aid - the only major economy to do so - Britain has always led the way when it comes to making this a safer, fairer, more prosperous world," Mr Cameron said.