Speaking on behalf of the Williams family, solicitor Robyn Williams said: "Our grief is exacerbated by the failure of his employers at MI6 to take even the most basic inquiries as to his whereabouts and welfare, which any reasonable employer would have taken."
He said the Williams family were "extremely disappointed" over the failure of MI6 to make relevant information available and asked for the Metropolitan Police to review how the investigation will proceed.
He added: "Gareth always was and always will be a special and adored son and brother.
"We miss him every single day and cannot describe the depth of the sorrow his absence leaves in our lives. We love you Gareth and will treasure your memory eternally. To live on in the hearts of those we love and leave behind is not to die."