At a state dinner in Washington, the Prime Minister likened Mr Obama to Theodore Roosevelt, one of the most respected and reforming presidents of the 20th century.

Mr Cameron said that Mr Obama had “pressed the reset button on the moral authority of the entire free world”. The comments were interpreted by the official White House reporter covering the dinner as “essentially an endorsement of another four years of Obama”.

Prime ministers typically do not endorse candidates in presidential election campaigns. Mr Cameron’s praise for Mr Obama, who will fight for re-election later this year, risks infuriating the Republicans, the party traditionally aligned with the Conservatives.