A “substantial phase of operational activity” in the search for missing Madeleine McCann is due to begin in Portugal.
Assistant commissioner Mark Rowley from Scotland Yard said officers are working through every credible line of inquiry in the search for the missing three-year-old, as part of the "slog of a major investigation".
He said: "In the forthcoming weeks we are going to be going to a substantial phase of operational activity on the ground in Portugal."
The senior officer stressed: "It's something that you would expect in any major inquiry.
"A thorough serious crime investigation works systematically through all the credible possibilities, and often in an investigation you will have more than one credible possibility.
"Therefore just because we're doing a substantial phase of work in the forthcoming week doesn't mean that it's going to immediately lead to answers that will explain everything."
British police are currently running their own investigation into what happened to Madeleine, who vanished from Portuguese holiday resort Praia da Luz in May 2007.
The Portuguese have now also re-opened their inquiry into her disappearance, and while they are working with the UK force have refused to set up an official joint investigation.
Mr Rowley echoed Kate McCann's calls for restraint in media coverage of the case, and said: "I want to be able to go back to Kate and Gerry at some stage in the future and tell them we've got to the bottom of this, or second best is to go back to them and say we've turned over every stone and we can't get to an answer sometimes."
A number of officers from Scotland Yard are hoping to be involved in the latest phase of activity.
"The activity in Portugal is led by the Portuguese, that's absolutely crystal clear in law," Mr Rowley said.
"We have some officers who would like to be helping with that on the ground in Portugal, doing some of the work we anticipate. We are putting the finishing touches to the plans to the Portuguese in the coming weeks."
He added: "I anticipate a substantial phase of activity in forthcoming weeks including Portuguese and British officers but the detail of that is still being finalised with Portuguese colleagues and it will all be under Portuguese leadership."
One line of inquiry for Scotland Yard is a lone male paedophile who staged a series of sex attacks on young British girls while they were on holiday in the Algarve.
They are looking at nine sexual assaults and three "near misses" on British girls aged six to 12 between 2004 and 2006, including one in 2005 on a 10-year-old girl in Praia da Luz, where Madeleine vanished two years later.
Hundreds of people have already made contact with police in response to appeals to try to find the attacker.
Earlier this month, Mrs McCann responded angrily to reports about potential excavation work in Portugal.
She posted a message on the Find Madeleine website to say: "There is an ongoing, already challenging, police investigation taking place and media interference in this way not only makes the work of the police more difficult, it can potentially damage and destroy the investigation altogether - and hence the chances of us finding Madeleine and discovering what has happened to her."
Mr Rowley warned that, after sifting through all the possible lines of inquiry, officers may draw a blank.
"There's lots of potential lines of inquiry," he said. "If we didn't think there were any potentially fruitful lines of inquiry, we wouldn't be where we are today; we would be saying there's nowhere to go with this investigation.
"There are many potential fruitful lines of inquiry and the only way you get anywhere is to work through them all systematically.
"Within that may be the answer and the case may be potentially solvable.
"We may be able to work through potential lines of inquiry over a period of time with the Portuguese and all of them draw a blank, that can happen."