Police issued a statement, saying, "At this early stage it is being treated as unexplained and there have been no arrests in connection with the incident."

TMZ reported yesterday ... a source connected with the death investigation told us police were "operating under the suspicion" it was an overdose.

Scotland Yard also said, "Police were called by London Ambulance Service to a house in Camden Square NW1 shortly before 16.05 hours on Saturday 23 July, following reports of a woman found deceased."

And as we reported yesterday, we were told when paramedics arrived there were "signs of life," though we did not have specifics.

Now the post-mortem is scheduled for Monday morning.

TMZ caught up with "American Idol" honcho Nigel Lythgoe who called Amy's passing "tragic."