Culture minister David Lammy has listed the ruin of the Temple of Mithras, which is now situated in an open-air site next to James Stirling's No 1 Poultry, as Grade II.
Lammy insisted that despite the fact that it was moved and rebuilt, it was still one of the most important Roman ruins ever found in Britain.
Lammy said: 'This temple is a highly significant remnant of Roman London representing the only known Mithraeum in the City.
'It has been a great archaeology learning tool in the heart of London and it will now be preserved for future generations.
'Listing is a tool for management of the historic environment, not a tool for its absolute preservation, and the proposal to relocate and reconstruct the Temple in its original site can now be considered in that context,' he added.