Scotland Yard stressed that the document, sent on Friday, was a routine part of their inquiry but would not reveal what the letter said.
The communications are used to ask the Portuguese police to carry out certain pieces of work on the British force's behalf.
Deputy Assistant Commissioner Martin Hewitt said that so far no requests to carry out interviews or arrests had been made.
He said: "We sent a further detailed letter of request which went out from the CPS on Friday.
"This is part of an ongoing process.
"The way this investigation is working at the moment is we are using that process, letters of request, and they are now going to be going out routinely."
Detective Chief Inspector Andy Redwood has travelled to Portugal again this week for a routine meeting with investigators there.
Mr Hewitt added: "Clearly the investigative tempo is moving forward as we're progressing the investigation and the work that we're asking the Portuguese to undertake for us.
"We're carrying on our liaison at all levels."
Madeleine, who was then nearly four, 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 into the youngster's disappearance in July last year - two years into a review of the case - and made renewed appeals on television in the UK, the Netherlands and Germany.
After shelving their inquiry into Madeleine's disappearance in 2008, Portuguese authorities said last October that a review had uncovered enough new information to justify reopening it.