Britain has not yet requested Portuguese help in its new inquiry into the disappearance of Madeleine McCann. 

Scotland Yard said a week ago that it was beginning a ground-breaking investigation and would have a team of detectives based on the Algarve, from where Madeleine went missing while on holiday six years ago.

Senior Metropolitan Police officers said they wanted to trace 38 potential suspects across five countries, had new lines of inquiry to pursue and new witness evidence to develop. Classifying the case as a criminal investigation, the Met said, would give its detectives “teeth” to interview witnesses and suspects, search properties and other locations and, if necessary, make arrests.

But seven days later the international Letter of Request necessary to start the Europe-wide inquiry has not left London and the Portuguese authorities are said to be surprised by the lack of progress. The Crown Prosecution Service, which must deliver the letter to the Portuguese judicial authorities, said its officials were still drawing up the complex legal document.

Prosecutors are understood to have received very little notice of Scotland Yard’s decision last week to publicise its shift from a review of the case to a full investigation. International criminal investigations in Europe are governed by two treaties: the 1959 European Convention on Mutual Assistance in Criminal Matters, and the Convention on Mutual Assistance in Criminal Matters between EU States of 2000.

The conventions set out the accepted procedures for exchange of legal papers, holding hearings by videolink, the operation of joint investigation teams and the control of undercover officers, phone taps and interception of e-mail and other communications.

Madeleine was almost four when she disappeared from her family’s holiday apartment in Praia de Luz in May 2007. She had been asleep in the apartment with her younger twin siblings while her parents, Kate and Gerry McCann, dined with friends nearby.

The original Portuguese police inquiry was inconclusive and was shelved in 2008. It cannot be re-opened unless there is significant new evidence that reaches a threshold set down by Portuguese law.

David Cameron ordered the Met to review the case in 2011 and a team was set up to examine documents gathered by Portuguese police, British agencies and private detectives hired by the McCann family. The discovery of new leads during that review convinced the Met there was a chance of solving the case.

Andy Redwood, the Detective Chief Inspector who is leading the inquiry, Operation Grange, said last week: “It is a positive step in our hunt for Madeleine that our understanding of the evidence has enabled us to shift from review to investigation.”

Despite 16 visits by British detectives to Portugal in the past two years, Lisbon insisted that “the British authorities cannot act on their own in Portugal”.

The Attorney-General’s office added that British police “will not be able to direct any interrogation or effect any investigation, on their own account, in Portugal”.

A spokesman for the CPS said: “Any legal communication between different jurisdictions requires extremely careful correspondence, including appropriate translations. We are in liaison with the Portuguese authorities, and will send the formal Letter of Request as soon as possible.”