Twenty fire engines and 120 firefighters took nearly six hours to bring the blaze under control on 6 June.
The hotel had just completed the “most extensive” multi-million pound refurbishment in its 115-year history, which included the installation of a “green living wall” surrounding its inner courtyard.
London Fire Brigade said in a statement: “The fire is believed to have been caused by the byproduct of arc welding landing on the felt lining of the planting facade.”
The 181-room hotel and its restaurants, which include Dinner by Heston Blumenthal and Daniel Boulud’s Bar Boulud, remain closed until further notice.
A total of 36 guests and 250 hotel staff were evacuated in the fire, with no injuries reported.
In its most recent statement the Mandarin Oriental said: “While it is too early to state when we will reopen, initial assessments have shown that the fire damage has mainly been confined to the exterior courtyard area of the hotel with limited impact on the interiors.
“We would like to express our thanks to the emergency services for their swift response, as well as our appreciation to everyone who has conveyed their concern about this unfortunate incident.
“We look forward to re-opening as soon as possible and will provide updates on a regular basis.”