The investigation into the disappearance of teenager Alice Gross is now a murder inquiry, police said, after they recovered a body.
Metropolitan Police Commander Graham McNulty said "significant efforts" were made to hide the body, which was found by officers searching for the missing 14-year-old in the River Brent, west London.
Mr McNulty said: "We are unable to make a formal identification at this stage, but clearly this news is devastating for everyone involved in the search for Alice.
"At this time my thoughts are with Alice's family and friends. I would ask you to respect their privacy and allow them space.
"This is now a murder investigation and I need the public's help to find whoever is responsible."
Alice went missing nearly five weeks ago after leaving her home in Hanwell, west London.
Mr McNulty said a search was carried out in the river last night as part of the investigation into Alice's disappearance.
He said: "Following this search we have sadly recovered a body from the water. This is obviously a significant development and Alice's family has been informed."
He urged anyone with information to come forward, adding: "Even if you have not yet spoken out it is not too late to tell us what you know."
Mr McNulty went on: "Our work at this scene is crucial to ensure we capture all the available evidence allowing us to identify who is responsible for this dreadful crime.
"This may take some time and I ask people to remain patient with us.
"I can confirm that significant efforts were made to conceal the body.
"At this point I do not wish to speculate any further on what has happened.
"Finally, I would like to reiterate my request that Alice's family and friends are given the time to come to terms with this news. My thoughts, and those of all of us in the Metropolitan Police, are with them at this difficult time."
He thanked the local community for their support and patience during the investigation, adding: "This discovery will have a significant impact throughout the borough.
"You only need to walk around the surrounding streets to see the effect that Alice's disappearance has had on the whole community."
The police search for missing teenager Alice Gross has been a huge and at times controversial operation.
Hundreds of officers have taken part in what is the Met Police's largest such inquiry since the 7/7 bombings in 2005.
In recent days the search has concentrated on scrubland along the towpath near the Grand Union Canal in west London, where the 14-year-old was last seen after leaving her home in Hanwell, west London, on August 28.
Divers searched the canal for clues in the early part of the investigation and by September 20 around 600 officers from eight forces had searched nine square miles of open land and 3.4 miles of canals and rivers.
A dedicated team of 30 officers have scoured hours of footage from hundreds of CCTV cameras.
As the search became increasingly desperate it emerged the RAF had been drafted in to help in their search for the schoolgirl, providing support with "aerial analysis" to pinpoint places to focus the hunt.
Last week, Scotland Yard said the investigation had so far followed 729 lines of inquiry, spoken to 1,067 people and had more than 1,000 calls, though there have been no confirmed sightings since her disappearance.
The search has also included a reconstruction of Alice's last known movements which attracted 150 phone calls from members of the public with possible information.
The Metropolitan Police's homicide and major crime squad assumed control of the search early in September but continued to stress it was still a missing person inquiry.
Two men were arrested in the days after Alice's disappearance but both were later released and told they will face no further action, before convicted murderer Arnis Zalkalns, who was filmed cycling the same route behind the teenager, emerged as the prime suspect.
Police have been working with their counterparts in his native country of Latvia to track down the labourer, who was reported missing within days of Alice's disappearance.
But the investigation has come under fire amid claims of delays in identifying Zalkalns as a risk, while detectives later admitted they have no power to arrest him if he has managed to flee abroad.
Commander Graham McNulty said the force would need to be "charge ready" to apply for a European Arrest Warrant, and has not done so because this would stop officers being able to interview the suspect.
The general labourer, who worked at a building site in Isleworth, west London, is thought to have come to the UK in 2007, but authorities here have faced criticism for apparently holding no record of his conviction for bludgeoning and stabbing his wife Rudite to death in Latvia.
A review is being carried out of the early stages of the investigation into Alice's disappearance.
The teenager's parents have insisted they have complete faith in the police investigation, and said they want the focus to remain on finding their daughter.
Police have not specified where the body was found. The River Brent runs a short distance from Hanwell and the towpath next to the Grand Union Canal where Alice was last seen on CCTV.
Mr Zalkalns, who was filmed cycling the same route behind the teenager, emerged as a suspect in her disappearance.
He was last seen at his home in Ealing, west London, on September 3.
Alice was last seen on CCTV walking along the towpath next to the Grand Union Canal as it passes under Trumpers Way at 4.26pm on August 28. She has not been seen since.
Her disappearance sparked a huge police search - the Met's largest since the 7/7 bombings.
Hundreds of officers from several forces around the country have taken part in the investigation. The Ministry of Defence confirmed the RAF was helping police in the search.
The inquiry has attracted controversy after delays in identifying Zalkalns as a suspect in Alice's disappearance, and detectives have admitted British police would have no power to arrest him if he has fled abroad.
The general labourer, who worked at a building site in Isleworth, west London, is thought to have come to the UK in 2007, but authorities here have come under fire for apparently holding no record of his conviction for bludgeoning and stabbing his wife Rudite to death in Latvia.
Police have been working with the authorities in his native country.
Alice's mother, Rosalind Hodgkiss, said last week: "Every morning, as Alice's disappearance grows longer and longer, brings new agony, new anguish."
One barge owner, who has been living on the River Brent for two weeks, said he could not understand why the body had not been found sooner in the shallow water.
The man, who did not want to be named, said: "It is terribly sad if they have found Alice.
"But I can't understand why they didn't find the body the first time around. Divers have been here before, the river is only 3ft (0.9m) deep and there is no flow to it, so maybe they missed it first time around."