Bagot was educated at Lady Margaret Hall, Oxford (MA). She was attached to the Ministry of Defence, performing work for both MI5 and MI6. She was the first person to warn MI5 that Kim Philby, MI6 officer and Soviet KGB double agent, had been a member of the Communist party. Philby's denial of this fact led to his eventual resignation from MI6, and his flight to Moscow. Bagot also wrote a definitive account of the Zinoviev Affair in which a forged letter, purporting to be from Grigori Zinoviev, president of the executive committee of the Comintern, urged the British working class to rise up in an armed insurrection. The publication of the letter led to the fall of the MacDonald Labour Government, and it has been suggested that MI5 or MI6 may have been involved in leaking the forged letter.