Theresa May has taken 13th spot in an annual ranking of the world's most powerful people - five places lower than her predecessor David Cameron a year ago.
Forbes magazine said that Mrs May was facing "a difficult task" in pulling the UK out of the European Union and may have to seek support from US President-elect Donald Trump, despite an "awkward" start to their relationship.
Mr Trump himself was ranked second on the list behind Russian president Vladimir Putin, who took the top slot for the fourth year in succession.
Mr Putin has exerted his country's influence "in nearly every corner of the globe" and "continues to get what he wants", said Forbes, adding: "With a likely ally entering the White House, Putin's power may go largely unchecked for years to come."
German chancellor Angela Merkel dropped a place to third, followed by Chinese president Xi Jinping, Pope Francis, US Federal Reserve chair Janet Yellen, and Microsoft billionaire and philanthropist Bill Gates.
Climbing above Mrs May after being ranked below Mr Cameron in 2015 were Google founder Larry Page, Indian PM Narendra Modi, Facebook chief executive Mark Zuckerberg, European Central Bank president Mario Draghi and Chinese premier Li Keqiang.
Explaining Mrs May's ranking, Forbes said: "May will soon be charged with guiding Britain's withdrawal from the European Union without over-exposing its economy, a difficult task that will be intensified by pressure from EU member states.
"May, an Oxford graduate, will perhaps look across the pond for support. Her party has received praise from US President-elect Donald Trump, though the leaders have gotten off to an awkward start.
"In their first telephone call after Trump's election, he casually told May to `let [him] know' the next time she stopped by the United States."
Asked whether Mrs May was concerned at her slide down the Forbes list, the Prime Minister's spokeswoman said: "I don't think it's something that has been distracting the Prime Minister today from governing the country and preparing for a European Council."
According to the magazine, individuals are ranked according to how many people they have power over; the financial resources they control; if they have influence in more than one sphere; and how actively they wield their power to change the world.