"We will do nothing to jeopardise the effectiveness of these appeals and we have repeatedly stated that media speculation can be damaging to our efforts."
Madeleine disappeared from her family's holiday apartment in Praia da Luz in the Algarve on May 3 2007 as her parents dined at a nearby restaurant with friends.
British detectives launched a fresh investigation in July this year - two years into a review of the case - and a new BBC Crimewatch appeal on the case is to be aired on Monday.
The appeal will include a reconstruction of the "latest, most detailed understanding" of the events around the time Madeleine went missing.
Scotland Yard has revealed a vast log of mobile phone traffic could be the key to finding out what happened to the girl.
Madeleine's parents have said they remain "optimistic" of finding their daughter and will not accept she is dead until they are presented with clear evidence.
The Metropolitan Police now has a team of six Portuguese detectives based in Faro who are carrying out inquiries on its behalf.
- PA