Built in 1880-84 in a neo-Baroque style with a deliberate 'Italian Renaissance' feel, it has a large dome and cupola standing 61m (200ft) tall.
The ornate interior is breathtaking, with a nave wider than St Paul's, pilasters and columns in Devon marble, and more exotic marbles in the apse and altars.
Huge marble figures of the Twelve Apostles, along with most of its 17th century treasures, were brought to London from Italian churches.
Bizarrely, during the Cold War the KGB supposedly used the porch to drop secret communique's!