Safiyya Amira Shaikh, 36, entered a guilty plea at the Old Bailey on Friday.
She had also planned to bomb a hotel.
The charge states that Shaikh made contact with someone who could prepare explosives and went on a reconnaissance trip to scope out the historic site and a hotel as locations to plant bombs.
She prepared the words of a pledge of allegiance to the terror group Isis between August 2019 and October 2019.
She also shared terrorist documents via groups using the Telegram messaging app over the same period.
Shaikh, from Hayes, in Middlesex, wore a black robe and spoke quietly as she admitted preparation of terrorist acts and dissemination of terrorist publications before Mr Justice Sweeney on Friday.
Mr Justice Sweeney ordered pre-sentence reports ahead of sentencing on 12 May.
The judge ordered that the defence should serve any psychiatric report by 3 April.
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