The Metropolitan Police Commissioner has been accused of giving "unfair" attention to the Madeleine McCann case following his decision to review it.
Madeleine, who lived in Leicester, disappeared aged three while on holiday with her family in Portugal in 2007.
The Met agreed to review the case after a Home Office request, but London Assembly member Jenny Jones has said this was unfair on other crime victims.
Sir Paul Stephenson said he was keen to ensure that London "doesn't suffer".
He told a meeting of the Metropolitan Police Authority the force could receive requests on any case, including those outside London.
Campaigners and relatives of other lost and missing children held a vigil outside City Hall ahead of the meeting.
The Home Office said Scotland Yard would "bring their expertise" to the search for Madeleine.
But campaigners have raised concerns that this has happened at a time when police resources are stretched and the Forensic Science Service is due to close.
Ms Jones told the meeting: "A missing child is every parent's worst nightmare and I have real sympathy for the many families in this position.
"Because of the prime minister's request to review the Madeleine McCann case, the Met Police will be using a lot of time and resources to focus on one particular case, outside of their jurisdiction."
Sir Paul said he "jealously guarded" the force's independence and Met officers had previously been involved with inquiries in the Soham murders and alleged child abuse in Jersey.
He said the force always considered whether its expertise was "unique or unusual" and if it would be adequately compensated for its officers working on other cases.
He added that the government would pay for the inquiry.
Later Ms Jones said: "I am just not convinced by the commissioner saying that he has extra resources that he can move around so that other victims will not have unfairly lost justice as a result."
A Met spokesman said the review was being carried out at the request of the Home Secretary.
He said: "The commissioner has considered the request and the review will be carried out subject to funding being made available by the Home Office as the case is beyond the Metropolitan Police Service's jurisdiction."