The stainless steel columns of the memorial in Hyde Park, central London, were daubed with red and black slogans overnight with the messages "4Innocent Muslims" "Blair Lied Thousands Died" and "J7 Truth".
A spokeswoman for the Royal Parks said the slogans had now been removed after they were discovered early this morning by the park's manager.
She said: "We found it this morning. It has now been removed and the memorial can go ahead as planned. Obviously, we are very disappointed."
A picture posted on Twitter by broadcast journalist James Banks, for London Live, shows the full extent of the defacement.
The monument honouring the 52 dead in the attack on London's transport system in 2005 cost nearly £1 million and has 52 stainless steel columns, or stelae, 11.5ft (3.5m) tall. It was unveiled in 2009 at a memorial attended by the Prince of Wales, the then Prime Minister Gordon Brown and other political leaders.
The columns are grouped together in four clusters, reflecting the separate locations of the bombings - Tavistock Square, Edgware Road, King's Cross and Aldgate.
Four suicide bombers detonated their rucksack devices near these locations on the morning of July 7 2005, killing the 52 and injuring hundreds of others, some seriously.
Graffiti was scrawled on the memorial in 2009 two weeks after its dedication ceremony on the fourth anniversary of the attacks.