The government is planning to pass new laws to cut Britain's overseas aid budget, the BBC has learned.
It has raised fears among MPs that the reduction could be permanent.
There had been speculation the chancellor was proposing a temporary, one-off cut to help pay down the government's record deficit.
Foreign Secretary Dominic Raab said the UK is "a leading, if not one of the leading, countries on aid" and "that will continue".
The idea behind a temporary cut was to reduce aid spending next year to just 0.5% of national income, down from the legally binding target of 0.7%.
But the BBC understands that Rishi Sunak's reforms will require new legislation to be passed by Parliament, which MPs believe implies a permanent cut to the aid target or even its abolition.
The issue is that the 0.7% baseline for Britain's aid budget is enshrined in law by the International Development (Official Development Assistance Target) Act.
This does allow the government to miss the target in certain circumstances, such as if there is a substantial change in the country's national income.
Foreign and Development Secretary Dominic Raab is simply required to lay a report before Parliament explaining why he has missed the target.
But there is a growing belief at Westminster that this exemption can apply only retrospectively.
The act places a duty on Mr Raab to ensure the 0.7% target is met. If he misses it, the act requires him to describe what steps he has taken to ensure the target is met the following year.
Some MPs and charities believe these two provisions mean the government cannot declare in advance its intention to miss the target without breaking the law.
To cut the aid budget without fresh legislation might lay the decision open to judicial review.
MPs also believe that a one-off cut to the aid target - a saving of about £4bn - would hardly touch the sides of the £350bn deficit projected for this year. They say it only makes sense for the Treasury if the cut is permanent.
They also believe that if the government is going to reduce aid spending and face significant political and international criticism, ministers will be tempted to go the whole hog and scrap the target entirely.
Almost 200 charities, two former prime ministers, opposition parties, church leaders, ex-heads of the armed forces, global philanthropists have all come out against the cuts.
The Archbishop of Canterbury Justin Welby has urged Prime Minister Boris Johnson not to go ahead with cuts, saying that "helping the world's poorest is one of the great moral and ethical achievements of our country".
The risk for the government is that passing new legislation would give critics of the aid cut the chance to oppose and potentially block the changes in Parliament.
Although the government has a working majority of more than 80, it has seen a number of rebellions of late. One senior Tory MP said defeating the government on this "would be entirely doable".
Such is the scale of the reforms to the aid target that Mr Raab is expected to make a statement to MPs about it on Thursday.