It had been a crypt below St Stephen's Chapel and had fallen into disuse, being used at various times as a wine cellar, dining room for Speakers (who had holes bored into the wall to accommodate two kitchen chimneys) and (now unconfirmed by records) stables for Oliver Cromwell's horses.

Although much stonework was damaged in the fire, it was decorated in the 1860s by Edward Middleton Barry with gilded, painted and stenciled designs in rich colours to cover the walls, floor and vaulting.

In addition, peers and members of Parliament have the right to use the chapel as a place of christening[5] in the baptistery and font (whose basin was made from a single slab of alabaster) designed by Barry.

The body of former Prime Minister Margaret Thatcher was kept in St Mary Undercroft on the night before her funeral in April 2013.

[8] The honour was also accorded to the body of Tony Benn, the long-serving Labour politician, before his funeral in March 2014,[9] as well as that of PC Keith Palmer who was fatally stabbed carrying out his duties on the palace grounds during the 2017 Westminster attack.