The McCanns had hoped that police would be forced to lift their status as official suspects in their daughter's disappearance by Christmas, after DNA tests failed to find any evidence which proved they were involved.
But police instead told the public prosecutor in the case that they needed to speak to some of the couple's friends again.
Sources told the Portuguese news channel SIC that a team of detectives and the prosecutor would fly to Britain next week for the interviews.
They will deliver letters of appeal to British police, asking them to interrogate the friends again, and asking that Portuguese officers be allowed to be present in the interviews. The letters of appeal are legal documents required for the interviews to be carried out on British soil.
A source from SIC said: "Next week the police, the public prosecutor and the letters of appeal will be sent back to Britain."
The letters were drafted several weeks ago and initially asked for fresh interviews against the McCanns and their friends. (Daily Mail)