Theresa May has accused Boris Johnson of abandoning Britain’s “position of global moral leadership”, in her most unrestrained attack on her successor yet. Writing in the Daily Mail ahead of President-elect Joe Biden’s inauguration, the former prime minister had stern words about both the outgoing US president, Donald Trump, and her successor. May said that, in her view, Johnson has failed to honour British values by threatening to break international law during Brexit trade negotiations and backing out of the foreign aid target, writing that these two manoeuvres had not “raised our credibility in the eyes of the world”. “Threatening to break international law by going back on a treaty we had just signed and abandoning our position of global moral leadership as the only major economy to meet both the 2% defence spending target and the 0.7% international aid target were not actions which raised our credibility in the eyes of the world,” she wrote. In her article, May appeared to remind Johnson that he needed to live up to “our values” to have any aspirations for a truly “Global Britain” to play a key role on the international political stage, and urged him to adopt compromise. “We have been sliding towards absolutism in international affairs: if you are not 100% for me, you must be 100% against me,” she said. “Compromise is seen as a dirty word.” “We must reject a scene in which a few strongmen face off against each other and instead bring people together in a common cause. But to lead we must live up to our values.” May and Johnson have clashed repeatedly in the Commons over the past year and a half, particularly over Brexit talks involving Northern Ireland and the Good Friday agreement. Likening the storming of the US Capitol to “attacks on our own democratic institutions” such as the murder of PC Keith Palmer, who died during the 2017 Westminster terror attack, May condemned Trump for having “whipped up” a violent mob and described the election of a “decent” Biden as the next US president as a “golden opportunity” for Britain to become a force for good in the world again. “What happened in Washington was not the act of a lone extremist or a secretive cell, but an assault by a partisan mob whipped up by an elected president. I know from experience that leaving power is not easy – especially when you feel that there is more you want to do.” May was the first foreign leader to meet Trump in the White House in 2017. • This article was amended on 20 January 2021. An earlier version said Theresa May “was the first foreign head of state to meet Trump in the White House in 2016”. The UK prime minister is the head of government, not the head of state; and the meeting took place in January 2017. These errors have been corrected.