A group of Brexiters determined to end May’s leadership gathered in the plush office in Parliament on Wednesday afternoon to discuss tactics ahead of the no confidence vote set to begin just hours later.
Former Brexit minister Steve Baker - the shop steward of the European Research Group - was among those in the meeting, along with Duncan Smith’s old Maastricht rebel ally Sir Bernard Jenkin.
Duncan Smith has repeatedly claimed he would not submit a letter of no confidence to trigger a vote as he was subject to such an action when he was leader in 2003 - although he has said it is "time for a change".
By holding the meetings in his office, Duncan Smith is now revealed as a key player in the plot to oust May from Downing Street.
Other MPs in the meeting were Simon Clarke, Anne-Marie Trevelyan and Ross Thomson.
Asked about the meeting, Sir Bernard said: "I can't talk right now."
May will address her Conservative colleagues at a meeting in Parliament at 5pm on Wednesday, with voting on whether she should stay as leader beginning at 6pm.
A Downing Street source revealed May is set to tell MPs the confidence vote is not about who will lead the party into the next election, but securing a deal that delivers on the Brexit vote in 2016.
May needs to win a simple majority - 159 - to stay on as leader, and then cannot be challenged for year.
If she loses, the starting gun will be fired on a leadership election which May will be unable to stand in.
The results will be announced between 8-9pm on Wednesday evening.