The Foreign Office was alerted to fears over Gerry and Kate McCann by a British diplomat in Portugal just days after their daughter Madeleine went missing. The diplomat was sent to the holiday resort of Praia da Luz in the days following the four-year-old's disappearance and soon became concerned over "inconsistencies" in the testimonies by her parents and their friends.
After visiting the McCanns, the unnamed diplomat sent a report to the Foreign Office in London, admitting his worries about "confused declarations" of the McCanns' movements on the night of May 3.
He also noted the couple's "lack of co-operation" with the Portuguese police.
The diplomat's concerns were made over four months before Gerry and Kate were named arguidos (suspects) on September 7.
Contents of the letter were leaked to Belgian newspaper La Dernière Heure over the weekend.
The diplomat expressed his fears after receiving instruction from the Foreign Office to provide "all possible assistance to the McCann couple".
The French-language paper printed excerpts of the letter, quoting the diplomat as saying: "With the greatest respect, I would like to make you aware of the risks and implications to our relationship with the Portuguese authorities, if you consider the possible involvement of the couple.
"Please confirm to me, in the light of these concerns, that we want to continue to be closely involved in the case as was requested in your previous message."