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 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 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 resort Praia da Luz in May 2007 where she was on holiday with her parents Kate and Gerry, from Glasgow. The Portuguese have now also re-opened their inquiry into her disappearance, and although they are working with the UK force, refuse to set up an official joint investigation.
Mr Rowley echoed Mrs 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 hope 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. 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 from 2004 to 2006,