During a hearing at London's Central Criminal Court, Lewis Ludlow admitted preparing acts of terrorism and fundraising for the militant group.
Prosecutors say the 26-year-old wrote down his attack plans, saying Oxford Street was an "ideal" target because it was busy and "it is expected nearly 100 could be killed."
Police found the notes ripped up in a garbage bin and pieced them together. Ludlow's list of "potential attack sites" also included the Madame Tussauds wax museum, St Paul's Cathedral and a Shia temple in Romford, east London.
Evidence recovered from Lewis's phone included a video of him swearing allegiance to IS and pictures of crowded areas, which prosecutors said were taken during "hostile reconnaissance."
Ludlow, from Rochester in southern England, was stopped at Heathrow Airport in February as he tried to board a flight to the Philippines, where prosecutors say he planned to join IS militants. They say he later plotted to attack London, and allegedly set up a Facebook account called Antique Collections as a front to send money to militants in the Philippines.
Lewis, who was arrested on April 18, admitted preparing terrorist acts and funding terrorism.
Judge Nicholas Hilliard set sentencing for Nov. 2.
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