The International Development Bill was given backing from all parties in both chambers after being brought forward by Lord Purvis of Tweed.
It means that the UK's foreign aid budget will have to be at least 0.7 per cent regardless of the government or economic situation.
Christian Aid Chair Rowan Williams praised parliamentarians of all parties for coming together to pass the Bill.
"We have seen an impressive level of cross-party co-operation on this important Bill in recent months, with MPs and Peers from all major parties coming together to support it.
"Successive Governments deserve praise for raising UK support for the world's poorest people to this level," he said.
"Giving aid is an extremely practical way of demonstrating our love for our neighbour in need. I have seen the vital difference that UK aid is making to the lives of some of the poorest people in the world.
"This law will make UK aid more predictable and more effective than ever. We also know that aid alone is not enough. Now this law is in place, we should continue to focus on tackling the root causes of poverty, such as climate change and tax dodging by multinational companies in the poorest countries in the world."
The bill will now become an Act over the coming days.
The Church of England's lead Bishop on International Development, the Bishop of Derby, Rt Revd Dr Alastair Redfern, said: "The passing of this Bill through Parliament today is the culmination of literally decades of campaigning by churchgoers across the country.
"They have been motivated by their faith to campaign on behalf of the poorest people in our world and they see this as fundamentally an issue of justice. I hope that everyone who has been involved in this campaign feels very proud today.
"UK aid does a huge amount of good for the poorest people right across the world. From helping them to recover from disaster to fighting corruption, preventing disease and providing education, UK aid changes millions of lives every year."