Met Police Commissioner Sir Bernard Hogan-Howe said if the allegations of a smear plot against Stephen Lawrence's family are true, it would be a "disgrace" and the force would apologise.
He said: "I am personally shocked by the allegations that an undercover officer was told to find evidence that might smear the Lawrence family.
"The additional allegations that this was concealed from a public inquiry, and that Stephen's friend Duwayne Brooks was targeted, are also very serious. If these allegations are true, it's a disgrace, and the Metropolitan Police Service will apologise.
"It's imperative that we find out the truth about what happened as quickly as possible."
He added: "Undercover officers have achieved some extraordinary things in the fight against serious crime and violent disorder. But smearing the family of a murder victim would never be acceptable to me or my officers.
"We must now focus on examining these allegations of inappropriate activity so that the public can see that police are held to account for past actions."
Doreen Lawrence has said it will "take a while to gain trust" after she met with the Metropolitan Police commissioner over smear claims.
Scotland Yard has been confronted with another accusation of underhand tactics in its investigation of the murder of Stephen Lawrence.
The father of murdered teenager Stephen Lawrence said he felt "betrayed" by allegations the Met Police tried to smear his family.