Parvinder Singh left his foster parents a letter before going missing last Sunday.
In the note he says he is going to find his real mother and father because he wants to go back to India.
The 13-year-old boy, who lives in Bromley, has not been seen since.
Reproduced below is the text of Parvinder's letter.
The names of the foster parents and family have been omitted by the police.
"Hello J and K, Ab and As: "I am leaving home because I am going to search for my mummy and daddy.
"You all should not worry.
"If I find my parents, I will not return home.
'Helpless now'
"If I do not find them, I will definitely return.
"I was very happy with all these people at home.
"I am not leaving this home in anger, but I am helpless now - that I cannot go back to India without my parents.
"Therefore, I am now going to search for my mummy and daddy.
"I do not know where my parents are, but still I will try.
"Nobody knows that I am leaving this home, because if I had told you, you would not let me go.
"J and K, I am very pleased with you, because you gave me a lot of love and care.
"And I am very pleased with Ab and As as well.
"Ab gave me as much love as he gave to As.
"I will miss all of you very much.
"Ab, if you like anything from my bedroom, you certainly can have it, because you gave me a lot of love.
"It will be a gift to you from me."
Parvinder is described as Asian, around 4ft 8ins tall, with black hair which is styled in a quiff.
When he was last seen he was wearing a blue T-shirt with a Nike tick on the front, blue jeans and black Nike trainers.
Anyone with any information can call Bromley CID on 020 8284 8859, or Crimestoppers anonymously on 0800 555111.
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