An historical child abuse inquiry, which is also investigating the murder of three boys, is taking time but is ongoing, the Met Police has said.
The statement comes amid reports that Operation Midland, an inquiry into claims of child abuse by establishment figures, was not progressing.
Scotland Yard defended itself against criticisms officers had jumped to conclusions about the evidence.
It has also warned the media over the treatment of vulnerable witnesses.
Operation Midland was set up in November after a man known as “Nick” alleged that boys, including himself, had been abused by a group of powerful men from politics, the military and law enforcement agencies in the 1970s and 1980s.
It is also examining claims that three boys were murdered and has focused on the Dolphin Square estate in Pimlico, south-west London.
When the inquiry was launched, Det Supt Kenny McDonald – who is overseeing Operation Midland – said the officers who had spoken to “Nick” thought his account was “credible and true”.
In the statement released on Monday, the Met acknowledged the language used had suggested the force had pre-empted the outcome of the investigation.
“We must add that whilst we start from a position of believing the witness, our stance then is to investigate without fear or favour, in a thorough, professional and impartial fashion, and to go where the evidence takes us without prejudging the truth of the allegations,” the statement said.
It also said Operation Midland was a “complex case, where the normal avenues of evidence-gathering from CCTV, DNA and telephone data, are not open to us”.
“These cases take time, but the public can have confidence that allegations from witnesses will be investigated thoroughly,” Scotland Yard said.
The force said a newspaper journalist had, in recent weeks, shown the name of an accuser to someone who had been questioned by police over a sexual assault allegation.
It said such an action could be distressing to those who had made allegations of sexual abuse, lead to fewer people coming forward and could ultimately hamper police investigations.
“Names will be disclosed by police to those involved in the case, but that will be at the appropriate time for the investigation depending on how those lines of enquiry progress,” the Met said.