At the time, Full Fact questioned the accuracy of this claim:
While the UK is the most generous G8 economy in international aid relative to GNI, it does not give the most aid of any country in the world, neither relative to GNI or in absolute terms.
And today, in a two-sentence correction at the bottom of page two, the Mail admits:
A front-page article on 27 May said that Britain spends more on aid as a percentage of national income than any other country in the world. In fact it spends more than any other G8 country as a percentage of GDP and is second in the world behind the US in cash terms.
Despite the Mail's headline and article being shown to be untrue by Full Fact on the day it was published, it has taken the paper five months to admit it got it wrong.
UPDATE: The Mail's correction was the result of a complaint from Full Fact. Their description of the process makes for very interesting reading, particularly the attempt by the Mail to unilaterally publish a correction two days before the PCC was to adjudicate. Despite the PCC saying the original article was 'sloppy' and contained a 'significant inaccuracy' they did not agree with Full Fact that a front page correction was necessary.